My riding lawnmower won't start when warmed up

I’ve a riding lawnmower with a two-cylinder Briggs & Stratton engine. It starts up well when cold (with the choke applied), runs fine too. But if I turn it off (say, to refill the gas tank) and try to re-start it, no dice. I have to wait an hour or two before it will start again.

I’ve tried shutting it down by pulling back the throttle all the way (rather than simply turning it off), thinking it might be a flooding problem. Well, it may be – but that doesn’t seem to help.

The next time this happens, check the spark. My guess is that the spark is weak. You might try replacing the spark plugs. If this doesn’t do it, lay a plug on the engine block and connect it to one spark plug wire. Crank the engine. You should see a nice, fat blue spark and hear a definite snap. If you don’t, the problem is in the magneto. Years ago, I had the same problem with a Lawnboy mower. This was a 2 cycle engine and the spark definitely had to be good to ignite the fuel/oil mixture. I replaced the magneto points and condenser and this solved the problem. Your engine probably has a solid state magneto, so you don’t have points.

Next time this occurs pull off the air cleaner, and try to start the motor. If it starts then the air cleaner filter element is clogged. A clogged filter would allow the motor to start cold, but the mixture could be too rich when you go to restart it due to restricted airflow.

If it is a paper filter you buy a new one. If it is a plastic “foam” cleaner you wash it in solvent, dry it, and then apply some fresh motor oil to it and reinstall it.

My sons lawn mower has the same trouble. I called a local shop and described the trouble to him. He told me the trouble is most likely with the key for the flywheel. It is most likely bent, due to the blade hitting something, which causes the timing to be off a little. He said he replaces them all the time. We haven’t replaced the key yet but I’m sure it will correct the trouble. The tech was very sure it was the trouble.