I had the same problem with my 3W85 i was 18 months chasing a bad starting motor untill i checked the timing it was set at 8-10 degrees before TDC.from the factory i had to make up a special bracket to move the sencer 1 inch round the crankcase.
Once found it transformed the engine to a one flick starter and 500 more RPM.
So my suggestion is when you obtain a new or second hand engine just spend a small amount of time to check the timing, it could save a lot of money and swear words.