Mysterious problem with Briggs Quantum engine

[rich1608]

Hi I wonder if someone here can help with a mysterious problem on a Briggs Quantum engine I'm working on. I've got the head off for a decoke and this afternoon while turning the flywheel the crank stopped almost dead with the piston half way up the cylinder. I turned it back the other way and the same thing happened. The engine seemed unable to get past the half way up position but after turning the crank back and forth several times it suddenly overcame the 'obstruction' (it felt slightly stiff) and now seems perfectly OK like it was before the problem occurred. I'm completely baffled!

The only thing I can think of which might have caused this is I rotated the exhaust valve while it was fully open. I've had a good look through the sump plug hole and everything seems fine. I also flushed out the crankcase with kerosene and drained it through a filter and didn't find anything. There was nothing caught in the flywheel. Naturally I'm now concerned something is wrong inside the crankcase only everything seems OK now. Has anyone got any thoughts on this? Many thanks Richard

 

[bertsmobile1]

Usually this will be a nut or rock stuck under the flywheel but the Quantums don't have magnets inside to pick up stuff.
However as this is on a push mower I would be looking underneath,

 

[rich1608]

Thanks for your reply, the engine is off the mower on the bench so it's nothing to do with the mower. I will have a closer look at the flywheel tomorrow for evidence of anything I might have missed but I had a good look and couldn't see anything at the time.