Front cylinder missing when hot at 4K to 5K
I am helping a buddy with his 1992 Vulcan 750. I have worked on many Hondas and Suzukis but not many Vulcans.
Symptoms: Only after the bike has reached full (hot) operating temprature. In neutral or cruising holding the throttle 1/4 to 1/2 4,000 to 5,000 Rpms front cylinder breaks up popping and misfiring very very bad. These problems came with the bike. Runs great for the first 5 or 10 minutes. Front plugs look foul even though they are brand new rear plugs look great.
What we have completed so far: Compression check 155 psi both cylinders, replaced both air cut off valves in the carbs because one was torn as well as clean the carbs (ultrasonic), rejet from 132 and 38 to 138 and 40 cause the exhaust has been modded, checked the petcock vacuum hose for any sign of fuel being pulled through due to faulty petcock. Checked fuel tank vent it is clear.
Any advice would be greatly appreciated. To me it seems like an ignition problem but then again it feels lean when it happens. The lean idea doesnt make sense looking at the plugs. They are not wet but do have quite a but of soot on them even after 30 min. running and 2 miles riding.