For a more precise answer we need Make/Model/Year/Engine Model/Deck Size, and hours on meter if you have one.
Engine quits when it warms up or gets hot.
Let's assume the engine runs at least 10-30 minutes for this scenario. If it runs only a few minutes, it's a horse of a different color. The possibilities, in order of easiest to fix, are
1. Fuel problems.
2. Spark problems.
3. Compression problems.
Tracking Down Fuel Problems
The normal list of procedures for tracking down fuel problems, relating to starting the engine, are not the same as fuel problems which develop when the engine has been running awhile (more than 10 minutes). This problem can be related to a gas cap vent being plugged. If your mower quits, take the gas cap off and crank it. If it starts right up..there's your answer. The gas tank is drawing a vacuum. Clean the vent or replace the cap. Some folks have drilled a 3/16 in hole in the cap and stuffed it with foam. This is a workable solution and cheaper than a new gas cap. But what if that doesn't solve the problem?
Consider my own experience with a fuel problem that caused my engine to quit in the middle of a mowing job: I was mowing along on my LA145 (200 hours on it) when it slowed down and died. Before it died, it acted like it was losing power. It thought it was only firing on one cylinder. I waited about 20 minutes and it restarted after a lot of cranking and some priming. I made a couple more mowing passes; the same thing happened again.
I have mowed commercially for many years and know this mower was in top running condition. There were no engine misses, no backfires. It was not abnormally hard to start. The carburetor was clean, the coils were good, the plugs and fuel filter were recently changed. Lifting the hood, I noticed the fuel filter was nearly empty. I pulled the hose off the tank side of the fuel pump and saw there was no fuel flow.
Gas should gravity flow from the tank to the fuel pump, if the tank has a good supply of fuel. I wiped the hose off and blew air back into the fuel tank. It helps to take the gas tank lid off first. After I got a good flow of air going back into the tank (you can hear the bubbles) the fuel began to gravity flow back through the hose at a decent clip. This fixed the problem. There is likely silt build up in the tank and it settles at the output port blocking the flow of fuel.
The problem repeated itself several times last summer. Back at the shop, I used compressed air (turned down to 30 psi) to blow the line clear. The problem has not shown up in quite some time.
What if the fuel system checks out and doesn't solve the problem? Check the ignition/spark system.
Start with the spark plug. You CANNOT test a spark plug (without expensive equipment in your lab). Did you know that it takes more juice to create a spark under compression than it does at normal atmospheric pressure? So just because you have a visible spark from the plug outside the cylinder does not mean you have a usable spark under compression.
The ONLY answer to this end of the problem is to get a new one; even if the current one is not very old. It's recommended they be changed every year, but a spark plug can fail even with very few hours of run time. The spark plug is subjected to compression, high heat, explosive forces, constant vibration, and of course electric shock in the 15,000 volt range. Make sure the gap is set correctly on the new one, typically 0.76 mm (0.030 in)., but check your specs.
Why you should just throw in the towel and start with a fresh spark plug? Read this wonderful essay on spark and spark plugs. Village Science Magneto Spark Plugs
- Install new spark plug(s).
- Run your engine under load till it quits.
- Insert a spark tester between the plug wire and the spark plug.
- In a darkened shop or garage crank the engine and check the spark.
- Compare the spark to the results described in the Spark essay above.
- If the spark is weak or non-existent, you possibly* have a coil, or plug wire from the coil, that is going bad when heated.
Note: Coils may not fail completely at first. They can be complicit in a situation where a mower starts and runs for a while before dying. As it heats up the coil parts expand and cause the engine to shut down. It will not re-start until the machine has cooled enough to allow the coil to cool. This can be 30 minute to several hours; perhaps it starts again the next day only to repeat the failure.
*Why did I say possibly? Slim chance, but it could be an element in the electrical system that is grounding the Coil/Magneto kill wire when it heats up or vibrates loose.
If you have not already done so, pull the shroud, or fan cover, from the top of your engine and look for a heavy metal wheel with fan blades. This is the Flywheel. It has permanent magnets embedded in the side. As the magnets fly passed the coil, current is generated. Seldom do the magnets go bad, but you should feel a strong attractive force from the magnets while you hold a screwdriver about an inch from the magnets. If they do not seem to be very strong you might have to replace the flywheel. This is a very rare happenstance.
Follow your spark plug wire back to the small plastic/metal thingy, mounted next to the flywheel, now you are at the Ignition Coil; or just Coil for short. TWIN cylinder engines have two Coils; one for each plug. A coil generates the 15,000 volts required by the spark plug. The Coil must be a precise distance from the flywheel. This is the Ignition Coil Gap. This gap usually .20 mm to .30 mm (.008-.012 in) from the flywheel. To set this gap properly, the magnets on the flywheel must be right in front of the coil when you set the gap. Check your specs. If the gap ain't right the spark ain't bright.
With the exception of the kill wire designed to stop the Coil from working, the Coil functions entirely independent of the rest of the wiring systems on the machine. If you disconnect the kill wire and the coil/plug wire are good you will get spark to your engine. The Coil works all by itself; it does not need the battery or the key switched on or even connected.
Inside the Coil are two windings of wire and no moving parts. With shorted (melted together inside) windings you can still get a spark, but it will be weak. With open (burnt/broken) windings, you may get spark intermittently or not at all. There are also electronic components like diodes which may be failing. Coils are subjected to heat and vibration and do fail on occasion. Another coil consideration is the spark plug wire. Back in the day, we would replace all our spark plug wires when we gave our car a tune up; because spark plug wires do fail. The only way to replace the spark plug wire on a mower is to replace the Ignition Coil.
Understanding the above info, let's proceed. Disconnect the kill wire from the Coil and crank the engine.
- If the engine still does not start, and the coil is gapped correctly, replace the coil.
- If the engine starts and runs, your problem is likely somewhere in the safety switches and wiring that grounds the coil kill wire.
- Note: with a disconnected kill wire the engine may start, but when you turn the key off, or otherwise try to shut the machine off, the engine will keep on running. Just slip the kill wire terminal back on with the key switch off, otherwise ground the coil kill terminal, or wait till it runs out of fuel.
What if it's not Fuel or Spark? Compression is next up. Compression is sister to air flow. If your engine has a lot of hours on it your problem may be simple wear. This problem is the most complex. There are only two simple possibilities
- Start with a clean air filter
- It's time to adjust the valves.
If a new air filter and a valve adjustment do not solve the problem, and the problem is not Fuel or Spark, then you will need more complex tests and solutions.
- Perform a compression test. This can be done with a simple compression gauge threaded into the spark plug opening.
- If compression is out of spec, do a valve job. You could just do this without the compression testing if you don't have a gauge.
- Perform a crankcase vacuum test. For further info on this test post a question, "how do I perform a crankcase vacuum test? Be sure to give engine model. Poor results on a crankcase vacuum test indicate the follow possible problems:
- Breather reed valve clearance and condition
- Seals and gaskets
- Valve covers gasket leakage.
- Rings, piston and cylinder wall wear/damage.
- Valve and valve seat wear/damage.
- Head warp.
- Perform a cylinder leak down test. This test checks the piston rings and cylinder walls for seal. For further info on this test post a question, "how do I perform a cylinder leak down?" Be sure to give the complete engine model.
- If you have a Twin Cylinder engine perform a cylinder balance test. This test is easier and takes less specialized equipment.
If an engine is hard starting, runs rough, misses or lacks power, run a cylinder balance test to determine if both cylinders are operating to their full potential. You will need an engine tachometer, a spark tester, and an insulated screwdriver.
- Using the spark tester, with the plugs in, start engine and verify spark is equally good on both cylinders.
- Remove spark tester.
- Connect engine tachometer.
- Run engine at top no load speed. Note rpm.
- Ground one cylinder spark plug with screwdriver. Note rpm.
- Ground other cylinder with screwdriver. Note rpm.
The difference in RPM between the cylinders should be less than 75 rpm. If the difference between the two cylinders is greater than 75 rpm, the cylinder with the least rpm loss is the weakest and is a potential problem.