take the plug out. if it turns over fine with the plugs removed but won't turn past compression then the decompression system on your mower is not working.
The usual culprit is the valve lash opening up and before you adjust it make sure the pivot stud is tight as they tend to work loose
One of the very few places I actually use loctite .
Other reason is excessie wear on the cam or cam follower, a lot more expensive.
Well the first thing we need to know is the model number of the engine so we know what we are talking about. Twin 2 would tend to indicate that it is a flat head and would not have any "Pivot" bolt and that it has a passive compression release which almost never fail.
