Bought a 1990 dt125r for my son to get about on, have deristricted it and fully opened the powervalve. Bike seems to missfire about 7k? anybody got any ideas why this is happening and how to fix it?

Many thanks

Have you tried with a new plug?

yeah got new plug in and now cleaning out exhuast and putting new exhaust gasket in, still has me baffled though. Might try a hotter plug as it seems to get better as it gets warmer, also its a pig to start???

Was it running ok before you desrestricted it and altered the powervalve?

Not really but as it was restricted it peaked at about 7k so i thought the missfire was probably due to the restriction?