It's been a week since I installed my big bore kit and I am now having some engine problems; normal use is fine but under hard acceleration, speeds above 50mph or at WOT I get what I think are missfires and I cannot go above 55mph whereas I could do 65 - 70 with my 125 cylinder. I'm hoping that it's just because the piston rings aren't seated yet but I fear that it could be something more serious. I have upjetted the carb a bit but I couldn't feel much difference.

Can someone help me out please?