I am responding to mainly subscribe. I have had two motors that I have not solved this problem. Two were solved by polishing the crank where it rubs on the crankcase, not the bearing journals. I have a couple MDS / Uchtkam motors which have too much clearance there and fuel dribbles out.the front bearing, so they had some quality control problems there. You will know this is the problem by touching the bottom of the case to feel if it is hot. It should only be a little warm. You could try taking out or adding a head shim too for more or less compression. I think maybe the newer motors have a bit too large of a carb hole than is needed. Maybe they were a bit optimistic of the power that that they could make. Maybe a sleeve in the throttle barrel to bring this diameter down a bit and check that there is good flow when opening the needle by blowing through the tubing and opening the needle a few turns past where it is running at. The muffler pressure fitting may have a too small hole as well. I had a bad batch of fuel this year that was doing that to a few motors. Some of these Russian and Chinese motors are puzzling at times. More of a kit. I have seen some nasty ones. Uktkam, CS MDS GMS and even T Tiger.. They can work great though when sorted out.