Ahh the joys of simplicity. I love small engines. There are so few things to fix! Of course, the details do matter but for a small engine to run, you really just need 4 things:
* Gas
* Compression
* Spark
* Timing
For car engines, the timing is generally set electronically. For small engines, it is also electronic (or perhaps points/condenser) but the weak link is actually the flywheel. If the flywheel key shears - which it will from time to time - the timing will be way off and the engine will fail to run.
Anywho, here are a few of my current thoughts on small engines. Basically the typical 4 stroke engine works like this: suck, squeeze, pop, flooey. OK, maybe my grandfather explained it to me like that 30 years ago but it's a great visual. Air and gas go in, the picture gets compressed, the spark make it go "pop", then the exhaust goes out and we start all over again.
If you want to make a dead engine run, the process is, check for gas, check for spark, check for compression, then check your timing.
Most often, it's gas. Gas won't go in if you've got a dirty fuel line or dirty carbeartor.
Next comes spark. The spark plug can be really fouled - full of smelly gas or oil - but in general, if it's not broken, the plug is not the problem. The easy test is putting the plug on the cylinder head then pull the crank through a few times. If you see a bright blue spark, the plug is good.
If you've got gas and you've got spark, timing is the next thing to check. If the engine stalled cuz the mower hit a rock or it backfired before dying, the flywheel may have lurch ahead and sheared the flywheel key. Remove the flywheel (sometimes a non-trivial task) and check the key. If you need to replace it, make sure you get one of the same type - don't use a steel one if the one you took out was aluminum. Also, when it's all done, be sure to re torque the flywheel nut to the engine spec.
OK, so lets say all that is good, what if you still can't start? Now it can get tricky. You may have compression issues - these don't just show up suddenly though. You may have bad oil - bad enough to make the engine not start. Pretty seldom though. So, if you've got spark, timing, and gas, you should be all set. If not. something has gone dreadfully wrong and it may be time to seek help.
Speaking of oil, I noticed something interesting recently. If the oil in your engine is old enough to suffer viscosity breakdown, your engine will run poorly when it gets hot. Essentially, the thin oil blows through important seals and causes the engine to stall. Play it safe and keep your oil fresh - one or two seasons of use max.
Small engines are fun and they even more fun when on a go-cart and it's moving you along at 40 or 50mph. Perhaps I'll go get a go cart.
