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
Obviously, the engine won't run without gas!
|NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Determining why it won't start
1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.
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- Is there some in the fuel tank? If it is near the bottom, add enough
so that there is no doubt about there being enough to reach the outlet
pipe regardless of any slant on which the lawn mower is located.
- Make sure any shutoff valve is open.
- Check for a clogged fuel filter, if there is one. There may be a sediment
catching screen at the bottom of the tank as well.
- If your engine uses a primer bulb, does it feel like it is doing something?
There is a distinctly different feel when it is actually squirting a little
gas into the intake pipe. Check that the rubber hasn't deteriorated but if
many pushes still doesn't do anything (and you're sure there is gas in
the tank and the engine hasn't flooded from TOO MUCH gas), the carburetor
and/or fuel line may need cleaning.
- If you are using gas from last season, discard it and start fresh. While
old gas will usually work in an engine in good condition, this is not always
the case, especially with one that has seen better days. The more volatile
fractions evaporate leaving behind higher flash point gas. Why add another
unknown factor to the puzzle?
- There may be water in the gas. If the carburetor has a drain plug, operate
it to rid it of the bottom layer which would have the water. If there is
no drain, repeated pulling on the starter cord should eventually clear any
reasonable amount of water.
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.
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 a small gas reservoir
which fills when the engine is off and provides some extra when starting.
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.
- Check that any choke is not stuck in the open position and not doing its
- The carburetor may need adjustment or cleaning.
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
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:
- Check the air filter. For testing, it can usually be removed to see if the
engine will start. However, do not run it for an extended period of time
without a properly functioning air filter in place. Some are designed to
be washed and reused while others must have their elements replaced.
- Check that any choke is not stuck closed. Though needed to start cold,
if the choke remains closed, the engine will not restart and will quickly
stop (truly choke!) due to an overly rich mixture.
- A defective carburetor may also cause the mixture to be too rich or too
WARNING: make sure there is no gas in the vicinity when performing the
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
If there is no spark, then there is a problem with your ignition system.
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.
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
In more detail:
- Check for a faulty or misadjusted STOP switch. This may be activated
by releasing the dead-man bar or by a throttle control lever (STOP, RUN,
START). Inspect the cable, linkage, and wiring for damage or for
something that may have come loose. Make sure you have the controls set
properly to run!
- Check that your starter is actually spinning the flywheel. If the
flywheel is not rotating properly when you pull the cord or turn the
electric start key, then there is a problem with the starter, not the
ignition system. Or, the flywheel is not tight due to a sheared flywheel
key or improperly torqued flywheel nut.
- Check for a flywheel that is loose and not seating properly on the taper.
This could result in no spark if the air gap between the flywheel magnet
and magneto core is then incorrect. However, due to the close spacing,
you would probably feel and hear serious scraping in this case.
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).
- Check for bad connections or defective wiring including faulty or
water logged insulation. If you just gave the mower a shower, wait
ample time for it to dry out. High humidity may result in more
problems if the insulation is not in good condition as well.
- Check for a weak (or missing) flywheel magnet. Both of these faults
are extremely unlikely unless you have been hammering and whacking the
crankshaft and flywheel in an effort to remove the flywheel. (This is
not recommended - see the section: Flywheel removal.)
- (a) Electronic ignition - There is likely a single potted module which
includes the circuitry and ignition coil. If anything goes wrong with this
module, replacement is the only option. Once the wiring and resistance of the
secondary has been checked, there are really no addition tests that can be
performed on an electronic ignition module without special equipment. A
defective ignition module will have to be replaced.
- (b) Breaker point ignition - Possibilities are bad, dirty, corroded, or
loose points or points that are grossly out of adjustment, a bad condenser,
or a bad magneto coil. See the section: Maintenance of
point-type ignition systems.
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 the blade hit an obstruction,
(1) or (2) are likely. See the section: Lawn mower will
not start after the blade hit an obstruction.
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