From my understanding, it can be 2 basic problems, fuel or spark. With the heat issue I would assume fuel , but that all depends on how hot thay are running.
To test for spark: while running, unplug the wire from the spark plug. The engine should sound different. Hold the end of the cord close enough to the spark plug to see a spark arc across, I would say about 1/2 to 1/8 of an inche away. try on all cylindars, one at a time.
If you don't hear a change in the engine when you unplug the wire, then that means no spark or no fuel.