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Copyright © 1994-2001
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Most problems with gasoline powered rotary lawn mowers as well as other small engines have simple and inexpensive solutions. For example:
For electric lawn mower and other electric yard tool problems, refer to the document: Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools which includes chapters on basic electrical theory (relax, no rocket science) and electric motor testing and repair information.
We will deal with problems the weekend gardener is likely to run into (sometimes, literally!) as well as semi-detailed overhaul instructions. One or more of the books listed in the section: References can be used to supplement this document and can provide much more detailed troubleshooting and repair procedures.
Even if you don't know the difference between a carburetor and a crankshaft, you may still be able to do some of your own work. (Hint: if you drop one of these on your foot, the crankshaft will hurt a lot more!) We begin with an introduction to small engine technology and have more-or-less separate chapters on basic maintenance, intermediate troubleshooting and repairs, and more extensive overhaul procedures.
I welcome comments, additions, hints, corrections, funny or other stories, etc. As the title implies, it is oriented toward the gasoline engine powered rotary lawn mower. However, much of the general information applies to a wide range of yard and shop equipment powered by small 2 and 4 stroke gasoline engines.
The following represent the basic precautions to take when performing maintenance or service procedures on gasoline powered equipment and dealing with gasoline in general:
Note: Modern mowers have at least two systems for stopping blade rotation fairly quickly and keeping it stopped if the dead-man bar is released. These will be either an engine kill and blade brake (on most inexpensive mowers) or a blade brake/clutch which keeps the engine running but stops the blade (on high-end machines).
Basic servicing of small engines doesn't require a $500 tool caddy. However, some basic hand tools and other items will be needed.
A basic set from Sears (Craftsman) should be fine and will come with a lifetime replacement warranty as well! If you have never invested in a socket set, now is the time. Forget about those $4 specials, however, as they are generally worse than useless. A word to the wise: you really must have a socket set to do any kind of work on small engines. Slip joint pliers or worse - ViseGrips - just will not do!
While box-end (closed) wrenches may be used for certain bolts, some simply are not accessible without a properly sized socket (like cylinder head bolts).
Electric equipment is in some ways more environmentally friendly generating no pollution (though the electricity had to generated somehow). Once the equipment is unplugged, there is nothing to worry about with no gasoline to store. Little maintenance is needed and there is never any issue of disposing of used engine oil - since there is no engine oil. Electric equipment is also usually - though not always - somewhat quieter.
The main disadvantage of line powered electric equipment is that it is tethered to an electric outlet by the power cord. This can become quite a nuisance after a short while. Battery powered equipment has tended to less powerful and more finicky to deal with than similar equipment powered from a line cord. And, electric mowers tend to be less powerful than similar equipment using a small gasoline engine.
Where your yard is relatively small (say, less than 50 feet to an electric outlet from the farthest point), a corded mower may be a good choice. It will be less expensive than typical battery powered mowers and most gasoline powered mowers, and vitually maintenance-free. Just make sure you use a proper outdoor heavy duty extension cord - probably one size LARGER (lower AWG wire size number) than what the manufacturer recommends. This will assure minimal loss of voltage due to its resistance - and every bit of power you have available will help! A somewhat lighter duty outdoor cord can be used for the first few feet if that makes maneuvering the mower easier. The main thing to watch out for is accidentally cutting the cord by running over it. Mowing in a back-and-forth pattern while moving away from the outlet helps. If you do cut the cord - don't panic. At most, you will need to shorten it a couple of feet and install a new socket on the end of what is left AFTER pulling the plug! If the outlet is now dead, at most you have tripped the circuit breaker or GFCI, or blown the fuse. Of course if you make a habit of this, your cord could get to be quite short. :-)
Battery powered yard equipment and power tools have improved greatly over the years. Some of the newer models are quite capable of cutting a modest size yard (e.g., 1/4 acre, manufacturers specifications may still be a bit optimistic) on one charge with ample power for moderately thick grass. But, there is quite a bit of variability in cutting performance and battery life so shopping around, consulting Consumer Reports, and making sure you get a return option if you are not satisfied are all well worth the effort - to save effort in the long run.
It is interesting, however, that quite capable battery powered tractors for example, have been around for a long long time.
See the section: Comments on electric mowers for more information.
Having said all that, the fact of the matter is that the vast majority of lawn mowers used for modest or larger lots are gasoline powered.
Of course, if you have a postage stamp size or even a small suburban lot, a manual reel mower may be your best choice - and you get some good exercise as part of the deal as well.
Also see the comments in the Chapter "Items of Interest" on electric and manual mowers.
Large mowers (those which carry you) may be of either the rotary or reel type, usually gasoline or diesel powered but some electrics have been produced. For information on riding mowers, lawn tractors, garden tractors and estate tractors (also known as compact diesel tractors), see the Small Tractor FAQ.
Consumer Reports regularly provides reviews and ratings of most common types of lawn mowers. These articles are a good place to start as they include a great deal of the basic information needed to decide on the lawn mower type best suited for your property. They compare a selection of typical models based on features, safety, price, and their tests of performance and operator convenience. If you do not subscribe to Consumer Reports, your local library will likely have access to several years of back issues.
For even more advice, see Consumer Reports Books' Yard and Garden Equipment Buying Guide. It is sold at bookstores and newsstands and is also available directly from Consumer Reports Books using the order form in the back of every issue of Consumer Reports.
The uniformity, consistency, and just general appearance of a lawn mowed with a rotary lawn mower is not quite up to the standards of that of a professional reel mower. You will never get the perfect manicured look though some models may come close. However, the simplicity, lower cost, and need for less and more easily performed maintenance will generally overcome the desire for perfection unless your lawn is featured regularly in "Better Homes and Gardens" magazine.
Gasoline powered rotary lawn mowers are by far the most common type used by homeowners and many professional landscapers as well. Most walk-behind mowers use a manual recoil (pull) starter though electric start is available on more sophisticated (and more expensive) models and generally standard on riding mowers and lawn tractors.
The main disadvantages of a gasoline powered mower are the need to deal with the handling and storage of gasoline and routine engine maintenance at the end of the mowing season to assure easy starting next season and to prolong engine life. However, most of this is pretty straightforward. See the section: General preventive maintenance. Small gasoline engines also do contribute to air pollution but new mowers must meet more stringent EPA requirements as of September, 1996.
Electric rotary mowers are also available in both plug-in and cordless (battery powered) versions. However, since gas mowers are generally more powerful and not limited by the length of a power cord or charge of a battery, they continue to dominate the market. Electric mowers are, of course, non-polluting but the electricity had to be generated somehow.
WARNING: Never defeat the dead-man control for any reason unless there is no chance of the mower starting.
WARNING: Rear baggers should never be operated without a bag unless the opening is fully blocked or the proper deflector is installed. Grass cutting performance will then be similar to that of a mulching or (side) discharge type mower respectively but since this is a compromise, the resulting appearance of the cut lawn may suffer.
Side discharge and bagging mowers can often be converted to mulching with the use of a mulching kit which includes a means of blocking the discharge port and possibly a special mulching blade. However, performance of one of these may not be as good as that of a mulching mower since the airflow requirements differ and these are largely determined by the design of the deck.
Above all, understand the very important SAFETY information.
If there is some 'simple assembly required', take your time and follow the instructions step-by-step. Despite the apparent efforts of the designers of the mower and the manual writers to make everything as obscure as possible, it will probably go together without undo difficulty if you use the proper tools. With some, all you need to do is unfold the handle taking care not to pinch any control cables - oops - and you are ready to go to work. Happy times are here again!!
IMPORTANT: For 4 stroke engines, make sure there is oil in the engine!!! Learn how to check it and fill it to the proper level if there is none or it is low. See the section: Checking the oil.
WARNING: Running an engine without oil can ruin it in a few minutes and your warranty will not likely cover such stupidity. Since the lawn mower may be shipped without any oil, it is your responsibility to check this and then add the proper amount of the correct type of oil to the crankcase!!!
For 2 stroke engines, special oil (not the usual 10W-30/40 type motor oil, nor WD40 or 3-In-One!) must be mixed with the gasoline in the correct proportions IN THE GAS CAN - not the fuel tank of the mower! See your users manual! Forgetting to use the proper mixture can ruin a 2 stroke engine in a matter of minutes and your warranty will not likely cover such stupidity. Adding the oil to the fuel tank is not recommended because thorough mixing cannot be assured.
If you have yard equipment with both 2 stroke and 4 stroke engines, clearly label the two gas cans to indicate which equipment each is used in.
For 4 stroke engines, gasoline is used as-is since there is a separate oil supply. For 2 stroke engines, you must mix the proper amount of the correct 2 stroke engine oil (outboard motor oil or whatever is recommended by your engine's manufacturer). Fill your '2 stroke mixture' gas can with about half the amount of gas you are preparing and add the proper amount of 2 stroke engine oil. Put on the top and slosh this around to thoroughly mix the oil in with the gas. Then add the remaining gas to the total amount for which your oil measurement was made.
How do you tell if a lawn mower you just inherited has a 2 stroke or 4 stroke engine? The vast majority are 4 stroke - look for an 'oil filler cap'. On many, this is clearly marked with words like 'oil' or 'oil fill' or with a suitably ambiguous icon. Removing it will reveal a dipstick. (Note that unlike the engine in your automobile, this is both the test and filler location.) However, on more basic models, it may be near the base of the engine and be unmarked. In addition, there will generally be markings as to the need for the gas/oil mixture somewhere on the cover. The only major manufacturer of lawn mowers I know of that has used 2 stroke engines in recent designs extensively is Lawnboy.
If you are unsure of the correct mixture ratio - they typically range from 16:1 to 32:1 gasoline:oil by volume - 16:1 is probably a safe choice. The worst that can happen is that the spark plug may be more likely to foul (and you will pollute more than necessary) but at least you won't risk damaging the internal parts from lack of oil. It is of course best to determine and use the recommended mixture ratio.
WARNING: When filling your gas can, place it on the ground a few feet from your vehicle - never fill it inside the trunk or truck-bed. See the section: Where to fill gas can.
However, a gallon of gas goes a long way with a walk-behind lawn mower. I don't have exact numbers but a gallon will probably do several acres of mowing (assuming you aren't chopping foot high grass!).
If you must transport gasoline in a vehicle, make sure the container is secured in an upright position and tightly capped (both the filler spout and vent openings). Provide adequate ventilation so there can be no possibility of fume buildup in the trunk or passenger compartment.
Also see the section: Explosion risk when filling a metal gas can.
(From: Joyce (joduren@ix.netcom.com)
Please be careful about filling the gas can. I saw something on one of those PBS How to do it shows (or was it in the newspaper?) that said that people are filling their plastic gas cans with them still in their trunks or hatchbacks. They said this is dangerous because the can isn't grounded or something and a static electricity spark could make the thing to explode.
"Isn't there a gasoline additive you can add to the gas which will keep it 'fresh' for up to 24 months? I've seen it at Home Depot, though have never used it. I think a tube was less than $1.00."
(From: Floyd Reed (floyda@ix.netcom.com).)
Being an old small engine mechanic from way back, I suggest that you don't use any additives to keep your gasoline 'fresh'. Dispose of old gasoline (end of season) by dumping it in the (nearly full) fuel tank of your car, then refill your gas can at the beginning of the mowing season. It should last you the season. With this method, you spend no extra on additives (that are no good for an engine), you don't waste 'old' gasoline, and you don't dump raw gasoline into the environment.
(From: Dan Weise (dan_yz@cin.net).)
I have found the commercial product Stabil the best thing since sliced bread. It prevents the gasoline in the gas tank from gumming up. I'm too busy to run around emptying gas tanks in the fall.
Instead, I shoot in a squirt of Stabil, run the engine for 5 minutes, shut the gas valve off (unless it's a suction carb mounted on the gas tank) and the engines start next spring.
That was *not* the case before Stabil. I can't count the number of times I've disassembled a carb and sprayed Gumout trying to unclog the main jet or the idle jet. Or had the float stick down and flood everything in sight. All because of gummy gasoline.
I've used Stabil for about 10 years with great success.
I also use that Slick 50 teflon stuff, but have no idea whether it does any good or not. But for the investment in an engine, I can't see that it hurts. The intermittent duty of many engines makes them more vulnerable to oil run-off than vehicles that are used daily.
In some cases, obstructions like tree roots cannot be moved. In this case, you will either have to mow around them or raise the cutting height of the blade to clear.
For the following, we will assume you pull a self-retracting starter rope. With an electric starter, replace the words: 'pull...times' with 'crank for several seconds'.
Move the mower or other equipment to the place where it will be used - no sense in dragging a chugging lawn mower through the neighborhood. Position it on a solid level surface. Make sure there are no loose stones, twigs, branches, logs, etc. underneath to get sucked up and thrown about once you succeed in getting the engine started (if you ever do).
When attempting to pull on the starter cord, it may be helpful to put one foot on the mower deck to brace it. Whether this is needed will depend on the design of your mower and in what direction the cord exits from the starter.
You or the starter motor supplies the power to get it started. However, at the low speed of starting, special modifications may be required to the fuel system for the engine to catch. These may take one of the following forms:
A typical starting procedure for an engine with a primer might be:
Automatic choke - temperature and engine vacuum control the fuel-air mixture.
A typical starting procedure for an engine with an automatic primer or automatic choke might be:
A typical starting procedure for an engine with a choke might be:
If the appropriate procedure is not successful, the engine may be flooded. You can give it 15 minutes or so for the gas to evaporate and try again or, if there is an IDLE or LOW speed position, open any choke and pull the rope several times in this position which should clear out the excess gas. Then repeat the recommended starting procedure.
If none of this works, you may have a starting problem and should refer to the section: Lawn mower will not start. Probably, you forgot to fill the gas tank!
Where behavior seems to have changed, first confirm that environmental conditions are the same and the gasoline is fresh before blaming the engine on starting problems.
More expensive equipment will have a blade brake clutch meaning that while there is still a dead-man bar but instead of killing the engine when released, it disengages the blade (clutch) and brings it to a rapid stop (brake). This is more convenient especially with a balky engine. There will then be a separate engine stop switch - possibly combined with a speed/throttle control.
Equipment with an electric starter may have an ignition switch just like an automobile and there will be three positions: STOP, RUN, START.
Some older equipment just has a stop contact that grounds the spark plug. Pressing on a lever connects the spark plug terminal to the engine chassis and kills the spark. While this is fairly reliable, it may be a momentary contact meaning that the engine may be on a hair trigger and even rotating the blade a fraction of a turn may cause the engine to take off again. Thus, disconnecting the spark plug wire or removing the spark plug is even more critical when working on this sort of equipment.
The most likely cause of such misbehavior is a stop wire that has become disconnected or has broken. This is easily remedied.
Also see the section: Additional comments on winterizing - draining vs. use of fuel stabilizer.
Here is another recommendation:
(From: Bill Harnell (bharne@adss.on.ca).)
Change the oil at the *end* of the season. No need to leave the acid charged oil in the crankcase over the winter to corrode the engine.
Then add a couple of teaspoons or so of Stabilit to the gas tank. Run the engine for approximately 5 minutes and while you're at it, inject some fogging oil through the carburetor to thoroughly coat all of the interior surfaces. Directions are provided on the fogging oil container.
Wipe the frame and handle with an oily cloth and oil all pivot points lightly. Clean the crud from under the deck - you do that frequently all summer - right? Remove all the grass clippings from around the flywheel and the cylinder fins.
Then store it in the shed or garage.
It will start on the first or second pull every spring.
BTW, you should be able to get both Stabilit and fogging oil at any reputable engine service center.
The proper amount of oil is critical to the happiness of your engine. Too little and it may overheat, cause excessive wear, and in extreme cases (but not unusual), cause engine parts to seize and fail - very expensive. Make it a habit to check the oil regularly. Doing this after about every 5 hours of operation is generally recommended. More frequent checks - such as before each time you mow - are fine as well. A typical small engine in reasonably good condition does not use up a lot of oil but checking oil is easy and will not hurt.
Oil should be checked when the engine is cold or after waiting 10 minutes for it to drain back into the oil sump after running the engine.
If the oil level is high - you just bought the lawn mower or were careless
in filling it last time - drain enough oil to bring the level back down to
the full mark. Too much oil can result in problems as well - oil spraying
out of various orifices or getting into other places where it should not
be like the combustion chamber.
If you find the oil level over the full mark or higher than it was before,
gas may be leaking into the oil due to a flooded carburetor - a stuck inlet
needle or bad float. If this is the case, the oil will need to be changed
once the underlying cause of the leakage is determined. (This is only likely
with float type carburetors such as those used on the Tecumseh engines used in
a variety of Sears/Craftsman models equipment.
If the oil is very low and you have been performing regular maintenance, there
may be a leak or your engine may need a ring job. Excessive oil under the
deck - on the shaft or blade adapter - would indicate a bad bearing or oil
seal. Noticeable blue smoke while running would indicate that excessive oil
is getting by the rings into the combustion chamber.
Typical oil capacity is just over 1/2 quart (usually about 1-1/4 pints).
The capacity of a typical small engine is just over a 1/2 quart. This will
probably cost you about 50 cents - a very worthwhile investment!
However, some people do swear by synthetic oil. I remember it worked pretty
well on my bicycle as well :-):
(From: Daniel Pope (dpope@l-a-net.net).)
Synthetic Motor Oil in the crankcase and Marvel oil in the gas is the only
way to go!
I have a $1,900 MTD tractor style mower with B&S engine (L head twin). It has
over 800 hours and 9 years on it and I mean rough hours (bahaya grass and hot
weather). The engine does not use a drop of oil (changed every 30 hours) and
the compression is the same as when new. These other guys can have there
$5,000 fancy mowers. I'll use that money to buy a truck.
Since there is typically no oil filter, all the grit, metal particles, and
other undesirable stuff continues to circulate with the oil to find its way
in between precision engine parts.
For small engines, the oil change interval is usually specified to be
about 25 hours of use. More frequent oil changes may be desirable if
the equipment is operated in an extremely dusty environment.
A typical mowing season for a modest size lot is around this amount of
time so an oil change once a season is probably satisfactory. I recommend
this be done at the end of the season so that the old contaminated oil
does not sit in the crankcase during the winter months and you will not
(conveniently) forget to do this at the beginning of next season when you
are eager to get at that straggly lawn.
First, drain the gas or remove the gas tank. If you will be filing steel,
you get sparks. Sparks are not the greatest thing to have around gasoline
vapor. Enough said. Disconnect the spark plug wire and tie it safely
away from the spark plug or remove the spark plug entirely. Turn the mower
on its side.
CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere.
If there are any, you will need to work on the mower just propped up by 45
degrees or so. Or, use this as a good excuse to perform an oil change and
drain the oil (even if the engine is cold, most of the oil will drain out but
it will take a little longer). Just don't forget to refill the crankcase with
fresh oil once you have completed work on the blade!
Check that the blade isn't bent. Locate a reference point on one side
and note the height of the blade tip at that location. Rotate the blade
180 degrees and check the height of the opposite blade tip. There should
be no significant difference - say no more than 1/8" or so. If it is greater,
the blade is bent or the crankshaft is bent. Either will require further
investigation as running the mower under such conditions will probably result
in excessive vibration and can be dangerous.
Assuming this is fine, inspect the blade:
Slight nicks and dents can be cleaned up with a file while the blade is
still installed on the mower. Unless you have run into a curb, this is
probably all that is needed on an occasional basis. Removing this small
amount of metal will also not unbalance the blade enough to worry about.
Refer to the section: Non-violent blade removal if
it needs to come off the mower.
If the damage is severe, consider replacing the blade entirely - they
are not that expensive (usually under $10). Otherwise, you can use a
file, a bench grinding wheel, or a grinding wheel mounted in an electric
drill (there are special attachments for this specific application).
Since the rotating blade also contributes to the proper air flow, you do
not want to upset the shape. Grind in such a way that the original blade
angle is preserved. It doesn't need (or want) to be razor sharp. A 1/64"
edge is fine. Anything finer will quickly be dulled by little bits of stone
and dirt in any case. Safety is not the main concern here - if any part of
your anatomy contacts the whirling blade, you **will** be in trouble no
matter how dull or sharp the blade might be!
Attempt to remove approximately equal amounts of metal from both ends and
in roughly similar areas if possible. If there are a few large nicks, it
isn't necessary to remove them completely - your lawn (and neighbors) will
never know the difference.
Check the balance by positioning the blade at the center hole location on a
pencil or other rod - you don't need a fancy blade balancer but can use one
if you like. If it tips one way or the other, remove more material from the
heavy side a little at a time.
Replace the blade along with all its mounting hardware. Make sure you get
all parts in the same relationship as they had originally. The blade must
have its sharpened edges pointing downward. Don't forget the install the
key if it is separate and DO NOT substitute a hard steel key for the soft
metal one that should be used. See the section: Why soft
metal keys must be used. If the locking key or blade adapter key appear
damaged in any way, replace it.
In either case: disconnect the spark plug wire and tie it safely away from
the spark plug terminal (several inches minimum) or remove the spark plug
entirely to prevent accidental starting.
If the nut holding the blade on is just on very tight, use a block of wood
to prevent the blade from turning. Use a good quality socket wrench or
box-end wrench of the correct size - an adjustable or even open-end wrench
may not be enough. The nut usually unscrews counter-clockwise. However,
check this out first! A careful inspection of the threads on the end of
the crankshaft will reveal the direction. Or, determine the direction of
rotation which will be designed to tighten, not loosen the blade. Most,
if not all, single blade mowers rotate the blades clockwise as viewed from
above which will therefore use a normal right-hand thread nut.
CAUTION: Make sure that if the wrench slips, your flesh will not contact the
blade or other sharp sheet metal - liberal use of rags or newspapers is a
good idea. Arrange your position and the mower so you are pulling towards
you - this is a more stable controllable arrangement.
(From: Graduate student of school of hard knocks.)
Use some penetrating oil (e.g., liquid wrench or WD40) on the nut and
threads if there are signs of rust or corrosion. Allow it to soak in
for a few minutes before attempting to remove the nut.
You will prevail. A hammer or other more violent approaches should not
be needed.
Once the nut is loose, unscrew it the rest of the way by hand and remove
any washers or mounting plate and note their exact position and orientation.
The blade and adapter should come off easily. Some penetrating oil (e.g.,
WD40) may help if it is difficult to remove.
If your adapter/blade doesn't pop off after removing the nut or bolt, it may
be mounted using a taper like the flywheel. This is somewhat unusual on a
walk-behind lawn mower but might be present on a larger machine like a lawn
tractor. A wheel puller is best for dealing with this situation but first
see if it isn't just gummed up or rusted in position - try the WD40.
Inspect the key or locking tab for damage. You may have:
If the adapter's tab is broken off or the key is sheared or damaged, then
replacement of the entire blade adapter or just the key (depending on your
mower's design) will be needed upon reassembly. For now, if you will be
sharpening the blade, replace all the hardware in the correct positions
(except the blade) and finger tighten the nut so you won't lose anything.
WARNING: Do not install a hard steel key in place of the recommended blade
lock key as you will lose the protection that the soft metal provides and
the next incident may be the last... See the section:Why
soft metal keys must be used.
Once you have reground the blade or obtained a replacement, reassemble in
reverse order and then tighten the nut to the proper torque.
(From: Gib Gahan (gahan@esinet.net).)
Another way to remove a stubborn blade is to take it to your friendly garage
or tire changer and have them put an impact wrench on it. Saves knuckles,
tempers, etc. Just don't put the blade back on without a touch of oil or
anti-seize compound and of course, don't use an impact wrench!
For adjustable types, it is best to refer to your engine manual. However, here
is the general procedure. Some of the specific numbers may differ for your
engine, however.
In all cases, before touching any adjustments, make sure you air filter is
in place, and clean (or new). Fill the fuel tank about half full with fresh
gasoline.
There are three adjustments on a typical carburetor:
Initially, carefully and gently turn the two mixture controls in until they
just seat.
Note: "In" means clockwise (the way you would tighten a normal screw) and
"out" means counterclockwise (the way you would loosen a normal screw).
CAUTION: do not force them - you are not trying to tighten anything - as you
will damage the needles and seats which will require replacement of the
needles or entire carburetor. Then back them out 1 to 1-1/2 turns. Set the
idle speed screw 1 to 2 turns beyond where it contacts the throttle plate.
Refer to your engine manual for specific recommendations! These settings
should allow the engine to start and run, though perhaps not entirely smoothly
or with great enthusiasm.
WARNING: if in doubt, set it low. It is better to end up with a scraggly
lawn than bodily injury or a blown engine! Note that by ear, 2 stroke
will always sound faster than 4 stroke engines for the same output
speed because they have twice as many explosions per rotation of the
crankshaft!
(From: (Willjim@gte.net).)
Once the air filter is saturated it must be wrung out. I typically place them
in a paper towel and squeeze. Then possibly a second paper towel. No oil
should be dripping out of the filter when gently squeezed. Chilten's B&S
engine section says to simply squeeze the filter of excess oil - no mention of
a paper towel, etc.
In adjusting the carburetor, Chilten says about 1-1/2 turns but 2 turns is my
standard.
I get the engine running first - warm it up to general operating temp. No
applied choke, air cleaner installed. Adjust the main jet if you can get it
to run at rated speed - preferably under load (turn in to the lean studder,
out to the rich studder then back in about 1/2 way between these extremes).
Then adjust the idle, at idle - at no load - using the same operation as the
main jet. You may then go back and repeat/refine the process a second time
as the idle adjustment may affect the main a bit.
For the 92000 in particular (typical but refer to your specific engine model
for exact specifications), from Chilten's second edition, "Repair & Tune-Up
Guide for Small Engines" (successor to [2]):
Since an automobile engine spark plug fires on every other revolution of
the crankshaft rather than every revolution as with most single cylinder
2 and 4 stroke engines, it will probably be necessary to multiply the
reading by a factor of 2. (Even though there is a power stroke every other
revolution for the 4 stroke engine, the ignition system is usually active on
every revolution. However, there are a few exceptions to this rule.)
For equipment where a shaft with a known speed ratio to the engine crankshaft
is available (i.e., a power take-off or trimmer head), an optical stroboscope
of one form or another may be used. You will just need to paint or tape some
stripes on the rotating part to put under strobe illumination:
For example, for something like a string trimmer which has a direct coupled
hub, strobe disks with 2, and 3, and 4 radial lines will appear stationary
under fluorescent illumination for 3,600, 2,400, and 1,800 RPM respectively.
There can be ambiguity but if you are already in the ball park, this sort of
approach may be all you need.
(From: Philippe Habib (phabib@netcom.com).)
Go to a hobby shop that sells radio controlled airplanes. Plunk down $30 or
so and get an optical tach. Paint 2 strips on the (crankshaft) hub of your
equipment to simulate a propeller, and you're done.
(From: J. Matthew Good (jmg14213@ix.netcom.com).)
You shouldn't need a tachometer on a trimmer. Two stroke engines in the size
and power range of line trimmers can't overspeed as they don't have the power
with a line head installed. Just set the carburetor up so that it 'two-fours'
at wide open throttle and you should be all set. If it needed to be adjusted
with a tachometer, it would have some kind of governor on it.
(From: Mowerman (mowerman2687@my-dejanews.com).)
B&S engines have a spring in the governor arrangement. You want to change
tension on the spring to change the speed. (This is basically true of most
other small engines as well but the details will differ.) The spring is
attached to a metal tung in the linkage at front of motor, this is made of a
tinny metal so you can easy bend this tung. To lower speed you want to lessen
the tension on the spring. You can do this while the motor is running at top
speed but it would be safer to do the bending while mower is not running. By
pushing in tung you will lessen top speed gently as it doesn't take much to
alter that speed. Often this tung gets knocked in while mowing around bushes
or other protruding material and "hey presto" your engine is only idling. It
is a design problem that briggs should be working on, however I love B&S
engines with their simplicity and ease of maintenance.
(From: foxeye@www.compumise.com).
I would start with them both backed out from the closed position to 2 turns
open, for starters. One should be idle mix and the other should be high
speed. Crank the motor, get it warmed up. You may have to fiddle with the
throttle and or choke until its warmed up. Then slowly turn in the low speed
jet, until it starts to die, then back it out another 1/4 to 1/2 turn. Then
hold the throttle wide open, and slowly turn in the high speed, until it
really starts to smooth out and rev high, and start screaming, then back this
out until it starts to run rough or slow down, and then turn it back in to
midway between these two positions. Keep playing with the low speed needle
until you get rapid immediate response from the throttle, and good idle wit
the idle adjustment. Then play with the high speed needle, but always back it
out from the setting about 1/4 turn or so, from where it runs the
smoothest. This setting of backing it out will allow more fuel when under a
load, and keep your 2 cycle motor from running too lean. Put a load on it and
just tweak the settings just a hair at a time. If your in the ball park it
won't take much adjustment either way to make a difference. Better on the rich
side than lean side. Also make sure your oil / gas is mixed at the proper
ratio. 99% of 2 cycle motors are adjusted the same, no mater what brand they
are. Most will start and run with both screws open 2 turns initially. Just
don't close the high speed off any more than necessary no matter how well
it runs.
While this chart lists many problems, it is does not cover everything that can
go wrong. However, it can be a starting point for guiding your thinking in
the proper direction. Even if not listed here, your particular problem may
still be dealt with elsewhere in this document.
(Portions of the following from: Chilten, Small Engine Repair 2-12 HP, (1).)
Note that the assumption here is that it cranks - the crankshaft and blade
rotates in a normal manner but the engine never catches. Some larger (Briggs
and Stratton) engines may have a low-oil cutoff switch which will stop the
engine if the oil level is inadequate. However, this is not likely on a
push mower.
Once you have exhausted these obvious problems, determine if gas is reaching
the cylinder as follows: Perform the normal starting sequence and then,
assuming it shows no signs of wanting to start, immediately remove the
spark plug. If fuel is reaching the cylinder, the spark plug should be
damp with gas and there should be a very distinct odor of gas from the
spark plug hole. If there is none, then there could still be a blockage
in the fuel line or the carburetor may need cleaning.
A flooded engine, most likely due to extended unsuccessful attempts at
starting or a defective carburetor (float valve stuck open or gas-logged
float) will result in inability to start as well and a distinct odor of gas.
You might find raw gas coming our of various orifices - air filter as well
as exhaust. (Note that in severe cases, enough gas gets mixed in with the
oil to significantly increase the level in the crankcase and reduce the
effectiveness of the oil. This will require an oil change.
To much air results in a mixture that is too lean, burns too quickly, and
can result in engine damage over extended periods of operation.
Too little air results in a mixture that is too rich - there will be loss of
power and possibly black smoke from the exhaust. This could be due to
several factors:
WARNING: make sure there is no gas in the vicinity when performing the
following test!
Remove the spark plug wire and insert the blade tip of an appropriately sized
and well insulated (plastic) screwdriver inside the boot or clip in place of
the spark plug. While holding the *insulated* part of the screwdriver,
position the metal part of the blade about 1/8th inch from the block or frame.
An alternative technique is to use an old, but good, spark plug whose gap
has been increased to about 1/8 inch or one specially made for exactly this
purpose. In this case, simply connect the spark plug wire to the test plug
and hold its threaded part against the cylinder head or other part of the
chassis (away from the gas tank!!).
Note: Just positioning the spark plug wire a short distance from the spark
plug terminal is not recommended as the results of this test will then depend
on the condition of the spark plug as well since the spark will have to
jump two gaps.
Have a buddy crank the engine at normal starting speed so that you will be
able to hold the screwdriver or test plug steady and be close enough to see
any spark clearly. Shield the gap from the sun or bright light if necessary.
You should see a nice healthy spark jump the gap several times on each pull
(actually, once per rotation of the crankshaft/blade on both 2 and 4 stroke
engines). Note: 4 stroke engines ignite the air-fuel mixture on every other
rotation of the crankshaft. The extra sparks fire harmlessly into the exhaust
gasses and are wasted. Can you believe it?!
CAUTION: if you are not well enough insulated, *you* will jump several times
per rotation of the crankshaft/blade if the ignition system is functioning
properly! Hey, that *is* a valid test!
If this test confirms the spark, it is still possible that the spark plug
is fouled or bad. See the section: Checking the spark
plug.
If there is no spark, then there is a problem with your ignition system.
However, a number of other problems can result in lack of spark:
Make sure stop switch/stop wire is in appropriate position - confirm with a
multimeter, check that flywheel is being spun by starter and that flywheel key
is intact to assure proper timing, check condition of points/condenser and
setting (if applicable), test magnet (on flywheel) for strength, check the gap
between flywheel and magneto core. If these are all fine, test or replace
the magneto.
In more detail:
Items (2) and (3) are likely if your just attempted to move a curb with
your mower blade (or if someone inadequately tightened the flywheel nut
during some previous maintenance).
See the section: Testing the magneto.
First, check that the dead-man bar is properly disengaging the stop switch
when pulled and/or throttle control is properly disengaging the stop switch
when in the start or run position.
For anything beyond this, disassembly will be needed to identify and replace
any defective parts.
If the no-spark condition happened after after the blade hit an obstruction,
(1) or (2) are likely. See the section: Lawn mower will
not start after the blade hit an obstruction.
The best thing to do at this point is just replace it with a new spark plug
and worry about the old one later. Actually, nearly every small engine
maintenance book will recommend changing the spark plug every season anyhow.
In an automobile, the battery supplies the primary current; in a magneto, the
magnet on the flywheel moving past the core at high speed acts as a generator
and induces current in the primary.
As the magnets spin past the pole pieces of the magneto core, the points are
closed and current builds up in the low voltage winding (and flux builds up in
the core). At or slightly before Top Dead Center (TDC), the current (and flux)
should be maximum and at this instant the points open. The flux then collapses
(and the condensor (capacitor) across the points acts as a snubber allowing the
current to bypass the open points and preventing arcing at the point contacts).
This rapid decrease in flux results in coupling of the stored energy to the
turn high voltage winding and results in up to 10,000 V or more at the spark
plug.
(For EE types, this is somewhat similar in basic operation to the flyback
converter in a switchmode power supply except that the moving magnet supplies
the input power instead of the rectified AC line and the points act as the
switch instead of a power transistor.)
The secondary will always be accessible for testing but the primary of an
electronic ignition may be not be due to the electronic components:
Wires can break due to corrosion or vibration. This would result in an open
winding - infinite resistance. Shorts can develop between adjacent windings
or to the core. This may be detectable as reduced resistance but without
knowing exactly what it should be, there is no way of knowing if a slight
discrepency represents a problem or just slight variations in design or
manufacturing.
A more complete test would involve checking the 'Q' or doing what is called
a 'ring' test and even more for an electronic ignition. This requires special
equipment. Therefore, it is best to swap in a known good unit. They are not
that expensive.
Timing is set on older mowers with point type ignition systems by adjusting
the point gap and generally only changes due to wear. However, these changes
are gradual and unless the points come loose for some reason, will not likely
suddenly prevent the mower from starting. On newer electronic ignition
systems, there is basically no adjustment as the position of the electronic
ignition coil/module fully determines ignition timing and this is fixed.
However, timing can be grossly messed up if the flywheel key gets sheared
and the flywheel then rotates a fraction of a turn on its mount on the
crankshaft. The result may be a mower that does not start, backfires or runs
erratically, lacks power, won't run and/or start when hot, etc. This is very
likely to happen should the blade strike a rigid object causing the mower to
stop instantly. In this case one or both of the blade lock key and flywheel
key have sheared to (hopefully) protect the very expensive internal parts from
damage.
There are likely not going to be any timing marks for that old timing
light you have sitting gathering dust somewhere. The only test really
is to inspect the flywheel keyway to determine if damage has occurred.
See the section: Lawn mower will not start after the
blade hit an obstruction as this is the most likely cause of a sheared
flywheel key.
The assumption is that the engine started and ran normally prior to
the incident. Now, no matter how many times you yank the starter rope
or run the electric starter, it will not start at all, bucks, kicks back,
backfires, or fails to develop enough power to keep going on its own.
If the blade struck a solid boulder while the engine was set on 'high',
more severe damage is possible as even with soft metal keys locking the
blade and flywheel to the crankshaft, the inertia of the rotating blade
is acting sideways against the crankshaft in addition to suddenly stopping
its rotation. This can result in a bent crankshaft. The end of the
crankshaft with the blade adapter could be bent without affecting the
bearings or internal parts. This would need to be tested for as well.
Not that such an occurrence is that much better - the crankshaft would
still have to be replaced but at least the bearings in the crankcase
will not be damaged.
If the starter will not turn the crankshaft (assuming you remembered in
your haste to engage the safety bar) - it is seized or will only rotate
part of a revolution before hitting against something solid inside - then
you probably have serious internal damage that will require a complete
strip down and replacement of some (expensive) parts. If it turns but
much more tightly than you recall (assuming you do have the safety bar
engaged!) then the crankshaft may be bent - again very expensive. Repair
may not be worth it.
However, in most cases, what has happened is that either or both of the blade
lock key and/or flywheel key have sheared to protect the crankshaft from
serious (and terminal) damage.
If the blade lock key broke, the blade will no longer turn rigidly with the
crankshaft and provide the inertia required by many small engines with
undersized flywheels. In this case, the engine may try to start but die
out with a few "putt-putts" or even kick back on the starter cord. (As a
side note, attempting to use a lawn mower engine as a replacement on a
piece of equipment that doesn't have something to substitute for the blade's
inertia may not work for this reason.)
If the flywheel key broke, the ignition timing will likely be totally wrong
and the result may be no ignition, backfiring, kickback, or weak or total
loss of power.
To diagnose, proceed as follows:
First, pull off the spark plug wire and tie it securely away from the spark
plug terminal (several inches minimum) or remove the spark plug entirely so
that there is no chance of the engine accidentally starting. Even though it
will not start now no matter what you do, the underlying problem could
actually be a flooded carburetor or something else which may correct itself
while you are working. Never take chances.
Drain the gas or remove the fuel tank. This will prevent gasoline from
spilling out the gas cap vent hole or flooding the engine through the
carburetor since you will need to tip the mower to get underneath.
Set the mower on its side (carburetor side up).
CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere.
The mower can usually be set on its side for a few minutes without harm but
if these occur - you will have to work with it tipped less than 45 degrees
or so - propped on wood blocks. Or, use this as a good excuse to perform
an oil change and drain the oil (even if the engine is cold, most of the oil
will drain out - it will just take a little longer). Just don't forget to
refill the crankcase with fresh oil once you have completed your work!
Using an old rag and/or proper work gloves, grasp the blade and attempt to
rotate the blade and crankshaft.
CAUTION, despite your lack of maintenance, the blade may be sharp!).
The blade and crankshaft should rotate together. If there is slippage, the
key has broken and will require replacement of just the key or the entire
blade adapter plate depending on design. If it appears to be intact, then
you can assume the flywheel key has broken. The blade key may be broken
as well but it is not likely the reason for your failure to start. You
should remove the blade to determine this for sure before restoring the
mower to service in any case. See the section: Non-violent
blade removal.
You can possibly avoid removing the flywheel for inspection of the key by
unscrewing the sparkplug, rotating the crankshaft so the piston is at TDC, and
noting the location of the magnet on the flywheel relative to the magneto
coil pole pieces. The magnet should be pretty close to the magneto in that
position. If this is not the case or just to be sure, the flywheel will have
to come off to inspect and possibly replace the key.
To get at the flywheel key itself, some disassembly is required.
You should now see the top of the flywheel. In most cases, a large nut
fastens the flywheel to the crankshaft. (However, in some designs, part
of the starter mechanism is actually used and this is supposed to require
a special wrench to remove. However, using a piece of wood as a buffer
and tapping the ears in a counterclockwise direction will work also. Refer
to your engine manual for details.) Use the proper socket to loosen this
nut (counterclockwise). It may be necessary to brace the flywheel securely
to gain enough leverage. Make sure this is done against something that can
stand the force. Once loose, remove it by hand and then remove any washers
or other parts that are under it. Make a note of how these were positioned
including which side is up on some cupped washers.
You should now see the keyway. The slots on the crankshaft and flywheel
should be aligned. There are two common types of keys:
You may even find that the flywheel is relatively loose on the crankshaft
if rotating the blade while holding the flywheel stationary is possible.
Either the blade key or the flywheel key or both are broken in this case.
You will have to remove the flywheel to replace the key if it is broken or
damaged.
If the flywheel is loose at this point, then the following will not be
needed as it can be lifted off.
There are several approaches to flywheel removal:
Bolts are screwed into holes in flywheel. Then, plate bears against the
flywheel nut (slightly loosened) and the nuts are tightened alternately
until the flywheel pops off.
WARNING: do not use an ordinary gear, clutch, pulley, bearing, or other
puller unless this is specifically mentioned as a recommended technique
in your engine manual. The flywheel could be damaged - possibly not
immediately obvious - but the result could be catastrophic failure once
the engine is put back into service.
(From: Foxeye (foxeye@www.compumise.com).)
The square shaft which turns one direction but not the other is the recoil
starter pawl. This can be pulled straight up from the round cup that it sits
in. There should be a couple of steel balls inside. Some models have a large
internal snap ring that holds this in the cup. Remove ring, and pull it
straight up. Use a magnet to remove the steel balls. Once you have this
starter pawl removed, and the steel balls, hold the flywheel, stationary, and
using a block of wood, bump one of the lugs on the outside of the starter cup
in a CCW direction.
Once you break it free of its torque, it should be able to be spun off
CCW. Remove this starter cup, and the beveled bellville washer under it. Now
your ready for your flywheel puller. A strap wrench can also be used to turn
this starter cup loose. When installing the starter cup, make sure it is clean
and free of grease and oils. A drop of oil or a light coating of grease on the
crankshafts stub end (over which the starter pawl goes) is usually
recommended. The starter cup only needs to be snugged down, and you don't have
to go overboard trying to torque it to ungodly tightness. Snug is sufficient.
Drop in the starter pawl, into the cup over the stub of the crank, and drop in
the steel balls, replace larger flat cover over this assy, and insert snap
ring if there was one.
Assuming there is no serious damage, a new flywheel key should be all you
need - about 25 or 30 cents. To confirm that this is all you need, replace
the flywheel without the key but line up the two slots as they would be
if a key were present. Tighten securely (but it doesn't need to be to
the full torque as this is just a test). This should permit the mower to
start and run normally but I would not recommend using the mower to actually
cut grass until you replace the flywheel key.
To install the new one, insert the key into the slot in the flywheel first
and then slip the entire affair onto the crankshaft (I like to use a bit
of WD40 for protection as well). The flywheel should seat securely with no
detectable free play - it should be on straight and not rock back and forth
at all. If this is not the case, the key may be in upside-down or there may
be something or some particles of dirt or metal blocking it. Replace the
washers, dirt screen, etc., and then hand thread the flywheel nut as far as
it will go. Tighten to the specified torque (typically, 30-33 ft-lbs).
Note: There may be a cupped washer between the nut/screen and flywheel.
This must be installed cupped-side facing the flywheel or else you will be
probably be replacing the flywheel key again very soon :-(.
WARNING: Do not install a hard steel key in place of the recommended flywheel
key as you will lose the protection that the soft metal provides and the next
incident may be the last... See the section: Why soft metal
keys must be used.
Then, replace the shroud, fuel tank, etc. If head bolts had to be removed,
it is probably a good idea to slightly loosen all of the head bolts and
then retorque them to the proper value in the recommended sequence for
your engine.
Note that the soft metal flywheel key can also be damaged without totally
shearing which may result in slightly incorrect timing. Symptoms may include
a mower that is hard to start, runs rough or lacks power, or cannot be
restarted when hot. Therefore, always replace the key if there are any signs
of damage or wear.
There can be numerous reasons for the engine to quit running when it
gets up to operating temperature.. expansion of metal parts where
bolts/screws aren't tight enough, breakdown of the condenser for the
ignition points (if equipped), or a possibility of a crack somewhere.
An overall inspection of the tightness of fasteners would be a good place
to start.
(From: Walt Conner (jerrbear@midwest.net).)
As soon as the engine shuts down, turn the flywheel by hand or DISCONNECT
the plug wire and turn the blade by hand to see if there is any
compression. I have had several B&S engines that did not have enough valve
clearance and when hot, the valve stem expands in length enough that the
valve does not properly close. After cooling, engine will be OK. Also could
be a bad condenser or the other faults listed above.
(From: Michael Stevenson (mike@gi4xsf.freeserve.co.uk).)
Problems with the ignition coil can make an engine impossible to start when
warm (or even stop running when it warms up), apparently this happens on
motorbike engines quite often and is caused by a break in the HT coil.
A carbon track forms inside the coil where the wire is broken. When the
engine is cold the carbon track has a low resistAbout engine oil
For the common 4 stroke engines such as Briggs & Stratton or Tecumseh,
you don't need to buy specially high priced engine oil. The type likely
used in your automobile will work fine and will be much less expensive.
Although small engine manufacturers may recommend SAE 30 oil, they usually
also state that 10W-30 or 10W-40 are acceptable substitutes - and these
are what your car probably uses. New oil should have at least the SG
rating though if you have some SF in your garage, that will be fine also
(for your lawn mower, not your car!). However, there are situations like
equipment that must run in extremely cold weather (probably not a problem
with grass mowing!) where specific alternative oil recommendations should
be followed.
Oil change
If you don't do anything else to prolong the life and happiness of your
small engines (and your automobile, for that matter) it should be an
oil change at the recommended interval. Oil loses its lubrication and
cooling effectiveness with use and this will gradually take its toll on
your precision engine parts. Even a simple lawn mower engine is machined
to very precise tolerances and any contamination will increase wear.
Ultimately, its performance - starting and running - will suffer and its
life will be shortened.
An oil change isn't really a big deal
The oil change procedure is a lot simpler than for an automobile as
everything is readily accessible and there is no oil filter to worry about.
Blade sharpening
The blade in a rotary lawn mower doesn't need to be sharp as a carving
knife or scalpel but serious dents and nicks will result in a less than
perfect lawn (to say the least!). In addition, a seriously unbalanced
blade can result in excessive vibration and eventually, possible internal
damage as well.
Non-violent blade removal
For just some minor touch up, there is no real need to remove the blade. For
major grinding and balancing, removal will be needed. Removal will also be
required to inspect for a damaged or sheared blade lock key and to replace
it if necessary.
"I'd wish I'd read this a few years ago. I used an open-end wrench and it
'flexed' off of the bolt. Needless to say, my next week wasn't a lot of fun
with 10 stitches in my hand."
Carburetor adjustments
If your engine is relatively new (made within the last 10 years or so) and
has only one speed, then there may be no adjustments (like the one discussed
in the section: Cleaning Craftsman (Tecumseh)
carburetors. It will either work or it won't - in which case it needs
cleaning or parts replaced. Or your overhaul was less than 100% effective.
You can tell if your carburetor is of this type as there will be no adjusting
screws on the carburetor. For Craftsman types, there will be a solid hex nut
on the bottom holding the float bowl in place. There may or may not be a
primer button.
Notes on Briggs & Stratton tune-up
(This assumes foam type air filter that needs have a film of engine oil in it
to trap dust.)
Setting engine speed
Many inexpensive mowers don't even have a speed adjustment screw. Spring
tension and the linkage to the governor set speed - period. Note that actual
speed is rarely that critical for engine driven equipment as long as it is
within safe limits. See the comments below on string trimmer speed settings.
However, what options do you have where there is a need to set the precise
engine speed?
Adjusting the carburetor on a 2 stroke engine
This is generic advice but probably a good place to start. It assumes that
there are 2 adjusting screws - idle and high speed mixture.
Troubleshooting Guide
Instant troubleshooting chart - most common problems and possible causes
The following chart lists a variety of common problems and nearly all possible
causes. Diagnostic procedures will then be needed to determine which actually
apply. The 'possible causes' are listed in *approximate* order of likelihood.
Most of these problems are covered in more detail elsewhere in this document.
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Possible causes:
Lawn mower will not start
This is probably the most common problem you are likely to encounter.
The cause is very often the same - lack of maintenance.
If you are unable to pull the cord (or the auxiliary starter on one with
electric start), there may be a clump of grass stuck between the blade and
the deck or there could be serious internal damage, especially if you just
encountered an immovable object. See the section: Lawn
mower will not start after the blade hit an obstruction. However, you
didn't forget to engage the dead-man bar, did you? On most inexpensive mowers
this safety interlock is needed to both enable the ignition system and release
the blade brake.
Determining why it won't start
Think: FAST - Fuel, Air, Spark, Timing. Diagnosing a balky engine is
not difficult but a step-by-step methodical procedure will make it a
lot less traumatic. Despite all the warnings, serious problems rarely
develop on their own. Most likely, there is a simple, easily remedied
cause.
Fuel
Obviously, the engine won't run without gas!
Air
The optimal air:fuel ratio is around 14:1. This must be lower for a cold
engine and thus a choke plate or other means to increase the richness of the
mixture is usually provided. A choke plate restricts air intake forcing
more gas to be sucked into the cylinder. A primer bulb effectively squirts
gas into the intake pipe to augment the normal carburetor action. Some
carburetors have no choke and no primer but incorporate an small gas reservoir
which fills when the engine is off and provides some extra when starting.
Spark
All common lawn mower engines require a precisely timed spark to ignite
the air-fuel mixture in the cylinder. The existence of a spark can easily
be tested as follows:
Lack of spark
If your mower is less than 15 years old, there is an excellent chance that
if uses an electronic ignition system. These are very reliable as there
are no points or condenser to go bad and no need for routine tune-ups.
Checking the spark plug
Use the proper socket to remove the spark plug and inspect it for damage and
general appearance:
Testing the magneto
The magneto, like the ignition coil on an automobile, contains two windings:
Timing
For power to be developed, the ignition of the compressed air/fuel mixture
must take place at exactly the correct instant - just before the piston
reaches Top Dead Center (TDC) on the compression stroke. With automotive
engines, there are mechanisms to advance the spark at higher revs but
simple lawn mower engines do not have this complication, at least.
Lawn mower will not start after the blade hit an obstruction
The following description applies to most small rotary lawn mowers with
direct driven blades. The vast majority of these use either Tecumseh (as
found a variety of Sears/Craftsman equipment) or Briggs & Stratton engines.
However, similar comments apply to others as well including Lawnboy two
stroke engines and the more modern Honda and other overhead valve type of
engines.
Flywheel removal
Remove the shroud (blower cover) if you have not done so already. This is
usually fastened with 4 screws and hopefully does not involve any head
bolts - if so, you will need to tighten them to the proper torque using a
torque wrench once you have remedied the problem. You may need to remove
the fuel tank (if you have not done this already) and other trim pieces as
well.
___ ___
|___| |___|<-------- Self tapping bolts or pretap holes.
| | | |
_|-|_ _|-|_
| |-| | | |-| |<------- Tighten nuts to release flywheel.
___|_|-|_|__________|_|-|_|___
| |-| |-| |<--- Plate or block - 1/4" or thicker steel
|_____|-|______________|-|_____|
|-| .-.----.-. |-|
|-| | :----: |<-|-|---------- Flywheel nut - loosen slightly.
|-| |_:----:_| |-|
______|-|___ :----: ___|-|______
//////:-:///|:----:|///:-://////
//////:-:///|:----:|///:-:////// <-- Flywheel comes with predrilled holes.
//////'-'///|:----:|///'-'////// (taper not shown - ASCII limitations!)
//////| |///|:----:|///| |//////
Flywheel removal on Briggs & Stratton engines
The following applies to most B&S engines with the square starter shaft. This
procedure will be needed to gain access to the points and condenser (on
engines without electronic ignition), oil seal, etc.
Flywheel inspection
Once the flywheel is off, inspect the keyway on the crankshaft and flywheel
for damage. Serious damage will require replacement of the affected parts.
Slight burrs can be removed with a small file. If there are any cracks in
the flywheel radiating from the hole, the flywheel MUST be replaced as this
is a serious safety risk - the flywheel could literally explode when run
at full speed. However, don't be concerned by surface flash - lines that
look like fine cracks resulting from the molding process. To confirm
that these are not cracks, there will be no visible penetration inside the
shaft hole and fine sanding will quickly remove all traces of this flash.
Why soft metal keys must be used
Normally, the soft metal keys lock the blade and flywheel to the crankshaft.
However, should the blade strike an obstacle and stop suddenly, one or both
key(s) will shear and reduce the likelihood that the very expensive crankshaft
or other parts will be damaged. The reason is that the substantial inertia of
the crankshaft and that of the flywheel will tend to try to keep them rotating.
Something has to give and you want it to be the 25 cent key and not the $75
crankshaft! However, this isn't foolproof as explained in the section:
Why you really don't want to attempt to move an immovable
object.
Engine dies or won't restart when hot
(From: Wild Bill (kwag98@tcis.net).)