Jabba:I would bet your local school 172's are not being started exactly as I describe. There will be all manner of throttle positions, mixture, priming and hence the problems.
I have no doubt you're correct on that score. Plenty of fried starters and flat batteries.
Jabba:The engine, provided it has good plugs, good mags and a F/A ratio that is right, WILL start every time. No argument. The pilot controls the F/A ratio and assuming the engine is "conforming to spec" all you do is get the F/A ratio right.
Agreed on having the F/A ratio correct.
I find with a hot start if it doesn't start after the first or second attempt you will need to prime. The amount of priming required can be hard to determine. On the next attempt after priming if there's not ignition after 4 to 5 blades I start opening the throttle to vary the F/A and almost always get a successful start.
Advancing the throttle on the first attempt at a hot start very often works as well.