The bottom line is that motor probaby needs to go into a shop. A worn ring, piston, cylinder or all of the above. Assuming you have the same type plug in each engine. You try a new plug, properly gapped, and see if that helps.
Tony, thanks for the quick reply. Problem is that it has done this since I had a new top end done. So thats why I was thinking maybe it might be something else, carb? Plug? etc…
It very well could be that it wasn’t rebuilt properly, which I hope is not the case, since it has done this since the rebuild and since it is my back up motor, that rebuild was done over a year ago, and probably hard to take it back to the builder and say he did it wrong. I did see him at the last race and asked him about it and he attributed it to my newbie buddy not driving it hard, but as I look back at it, I realize it has been this way with me driving as well, but it was so long ago, I just forgot.