could be the exhaust, a blown exhaust gasket allows air into the exhaust which allows any unburnt fuel to ignite, which it will at these temperatures, i.e. a backfire. also, check the plug and spark gap, the most likely cause of misiring. beat the plug contacts into submission with a wire brush (clean them). misfiring often means backfiring too, no ignition = fuel in the exhaust = backfire = pressure on the piston when its trying to come up = cut out.
steve, May 2005
