I asked about this recently, but wanted to start a new thread so it would get read by new eyes...
I have a John Deere JS46 that is hard to start. It has a Briggs 190cc OHV engine. When trying to start, all I get is pfffft, pfffft, pfffft, pfffft, pfffft, pfffft. It's hitting, and almost starts, but never quite catches hold. It backfires occasionally when trying to start as well. It started as a hot engine problem. Now it's the same cold. Someone suggested adjusting the overhead valves. I checked them, and they weren't far off, but I adjusted them anyway. It made no difference whatsoever, so I don't think that's the problem. I checked the flywheel key, and it is not sheared.
I understand that this engine has some type of system that relieves some compression during starting, to make it easier to turn over. I have no idea how this system works, but could it be the problem? I don't see any obvious signs of a blown head gasket, but maybe that doesn't mean there isn't one.
I'm good with mechanical things, but have little experience with small engines, and that's what is needed here.