well their isnt that much too it , you said you had the head off, did it have carbon on piston top and or cyl. head , I just had a 2 cyl. 18 hp that cranked hard also and what I did was finally sprayed water mist thru carb while on high idle to clean off piston and combustion chamber. or you might have too much timing advance, which I was told once if you set the coil gap from .030 to about .060 it would ****** the timing maybe 2 to 3 degrees. ,, just food for thought. I you voltage dropped and non over 1 volt while cranking and starter is new I think you stated ..that should be it . unless the top of cyl. bore is scored but that you would of seen with head off.. let me know.
