I pulled into a bud's place yeaterday--stayed for 30 minutes or so. When I tried to start the bike, it would run only on the rear cylinder. The front is getting fuel, but it's not burning--except occasionally when it ignites out through the carb. The bike was running great before I stopped. Pulled strong, no unusual noises, no missing, hesitation, or backfiring. The most recent plug check showed the plugs were "normal". No sign of running either rich or lean. What we know now: We are getting fuel into the cylinder--enough so that after turning it over a while, fuel leaks out the exhaust ports. We are getting good spark--confirmed instrumentally and visually. We have visually checked and replaced the intake gaskets. No indication of an intake leak. We are getting 150 lbs plus compression in the affected cylinder. We have visually examined the parts of the intake valve that can be seen through the intake port. Still shiny and clean--no signs of burning. Oh yes, the top end is new--it has about 150 miles on it.