Hi mate
i had one that wouldnt go above 6 and when i changed the plugs it seems to have done the trick. But then as it would rev properly the dizzy soon died as it must not of been used to spinning at such rates so beware of that, i had to replace that also. Mine kept popping and farting and cutting out.
Does it get to 5 grand them hit the limiter? might be worth changing the dizzy cap, rotar arm etc aswell.
