I have seen this happen to a few different motors over the years, usually it is a bad pinch bolt, what happens is, the 2 halves of the pinch bolt bottom out on each other before actually tightening up on the carb. It might even feel fairly tight when sitting there, but when everything heats up, it will expand and come loose. You could try a different pinch bolt, or just dremel a little off the face of the two halves of the pinch bolt, the parts that meet up, so it can "pinch" more before bottoming out. The other thing to check would be that the phenolic ring around the carb, if it has one, is not cracked, or tapered or something like that. If it is a one piece type pinch bolt, the end of it could be bottoming out on the nut before it pinches well. I think the only way you could fix that would be a new pinch bolt, or a washer between the nut and motor case, the washer would need a big enough I.D. to go over the pinch bolt itself. GOOD LUCK!