Hi JRC, it is something you have done or something you haven't done. I have done in excess of a hundred of these and can't say I have come across the problem you describe. I have had the occasional carby body that caused problems but if I have problems I just swap it out. The only reason it can go full throttle without control is because the poppet is opening too far, there must be something wrong with the cam or lifter for this to happen. As for swapping the "O" rings out I find that I probably only have to one in maybe twenty so they are very rarely an issue.From memory I have replaced only 2 "O" rings on the inlet manifold (I have never bought any) and I replace the pull start "O" ring only when I change pull starts. The primer cap "O" ring change is a must but that is an obvious one
