missfire

Hi Guys, I've got an 86 that's misbehaving on me and driving me up the wall. What's happening is that she'll run fine for 15 minutes to half an hour then starts misfiring like a horror. I think its being caused by something warming up because once she starts misfiring she wont stop, however if...

This is driving me crazy, does anyone else have this problem, and have they cured it ? At low rpm's between 1000 - 2000 occassionally one of my carbs backfires through it, usually when the bike is running on the hotter side, seems to be intermitent, but its annoyong - i have cleaned and synced...