a top racer who used to run the max told me the bearings sometimes go on these engines. check to see if there is up and down play on the crank.

also check the seals on your backplate, and carby. put a small bead of silicone gasket around the o rings to close it up.

also check your needle o rings. if it has been doing this fromt he word go, it may be the seals. maybe you didn;t notice this in break in because it was richer, and still being bedded in perhaps?