Answering Why Lawn Mower wont start is not difficult but a step-by-step methodical procedure will make it a lot less distressing. It helps to have guide as to where to begin when trying to diagnose any given problem.
I have found this section of a much larger article covering many aspects of small engines to be very helpful. There is a lot of good information here explaining Why Lawn Mower won’t start. You may want to bookmark this page.
Notes on the Troubleshooting and Repair of Small Gasoline Engines and Rotary Lawn Mowers
Version 2.39 (28-Aug-07) Copyright © 1994-2007 Samuel M. Goldwasser
--- All Rights Reserved ---
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.
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.
(Portions of the following from: Chilten, Small Engine Repair 2-12 HP, (1).)
Determining why it a lawn mower engine 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.
Obviously, the engine won't run without gas!
• 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 odour 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 odour of gas.
You might find raw gas coming out 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 job.
• 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 several factors:
• 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 lean.
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:
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 gases 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. If there is no spark, then there is a problem with your ignition system.
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.
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 multi-meter, 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:
1. Check for a faulty or mis adjusted 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!
2. 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.
3. 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 you’re just attempted to move a curb with your mower blade (or if someone inadequately tightened the flywheel nut during some previous maintenance).
4. 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.
5. 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)
6. 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.
6. (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.
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..
Checking the spark plug
Use the proper socket to remove the spark plug and inspect it for damage and general appearance:
• Light gray or brown and smooth - this is the normal appearance. The mixture is correct and there likely no major problems with the engine.
• Excessive black carbon - the mixture may be too rich or the spark plug may be the wrong type for your engine.
• Damage to the electrodes - the mixture may be too lean, timing may be set incorrectly, or the spark plug may be the wrong type for your engine.
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.
Testing the magneto
The magneto, like the ignition coil on an automobile, contains two windings:
• A primary with a few turns of heavy wire.
• A high voltage secondary with thousands of turns of super fine wire.
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 Centre (TDC), the current (and flux) should be maximum and at this instant the points open.
The flux then collapses (and the condenser (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 fly back converter in a switch mode 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:
• Secondary: 3 K ohms (maybe a little higher but not open). Much lower would indicate a shorted winding.
• Primary (if non-electronic and accessible): very low - guessing less than an ohm.
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 discrepancy 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.
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 Centre (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.
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.
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By having a basic guide to Why Lawn Mower wont start may save yourself many hours of anger and disappointment when you find out that it was something as simple as water in the gas that been preventing the mower from starting.
This information may also save you a lot of money by not having to take your mower right away to a small engine mechanic. Remember when ever you are working on your equipment think safety first.
I hope this helps why your lawn mower wont start.
“Keep It Simple to Succeed” lets get out there and make our lawns healthy and green!