Lawnmower engine woes

[Midwestcruiser]

10hp tecumseh lawnmower engine
will not start..just pops every 10-20 revolutions
has spark, fuel, air and compression(so it seemed)
took the head off and watched the valves by turning the crank by hand
it appears the exhauust valve bumps open for a second partway through the compression stroke
Any ideas on what the root cause is??

 

[D'Animal]

The exhaust valve partially opening during the compression stroke it the compression release.

It sounds like your timing may be off. This could be caused by a sheared flywheel key or one of the safety switches is not right.