THIS BAFFLES ME I have 17.5 kohler command motor on a john deere tractor very low hours ran fine shut it off tried to start it and started backfiring threw carb OK so I thought maybe it was a bad plug kept doing it so this is what I have done I checked compression reads a 100 lbs checked valves they are both opening fine thought it might be the flywheel key sheared that is fine is it possiable even though it has good spark that the coil is bad and throwing it out of time and also if you know what the valve clearecense are that would be helpful

I believe this has hydrulic lifters no setting.
Did you remove flywheel and look at key , if not you couldn't tell if it was sheared this has a half moon key in it.
Luck

I had it happen before to. The vaulves are sticking.Get some valve cleaner it will help