Is the gear box oil dropping in level as if the crankcase halves arnt sealed correctily is it blowing white smoke , what your fuel oil ratio mix and type of oil you are using, did you use a sealant where the o rings on the pipe that fit in the cylinder ,are your crank seals fresh what jets are in carby, are your reeds good , is the rubber manifold not cracked , is the power valve timed correctily and working , was it doing this before the rebuild , i would make sure the air cleaner not falling apart and even check your with in the timing marks while your there check if there any crank up and down movement. Make sure your got the right plug .is the muffler full of oil and needs new packing.Did you check ring gap and fit the rings the right way up etc , we need this info to cross of the list as it hard to tell what it is with out looking at it directily.hope this helps.
