Motorcycle Repair: 1978 cb400a, rubber diaphragms, stator coil

Question
Hi Bill

I have a 1978 cb400a with rejetted carb, air filter pods, and 2 into 1 exhaust. I replaced the cdi, stator coil, plugs wires, ignition coil and a lot of other things. I have an issue when the bike is warmed up. When I first start it and ride everything is fine. After a few minutes of driving it will begin to sputter at a light when I initially take off after I gain speed it fast and responsive. I try to pull out the choke all the way and it stalls but if I pull it out a bit it will be better at low speeds. I also think I have the idle set pretty hi because if it's to low sometimes it stalls. It revs pretty high in neutral. I can't tell though because I dont have a tach. I usually ride in first gear and leave it there while riding in the city. Also my air filter pod are the cheap ebay kind and I use the K&N cleaner with them which I'm not sure if that could be a problem. This all started after a bad rain storm and my airfilter pods got wet. I cleaned them and it seemed fine but maybe I was wrong.

Thanks

Carlos

Answer
Carlos, the carbs have dual air cut valves on them with little rubber diaphragms. I think one may be torn and creating an air leak/vacuum leak on one side.

Other possibilities are that the intake manifold is torn and causing an air leak, as well. Assuming that your fuel filter in the tank is clean and not restricting the flow to the carbs, then it sounds like lean condition issues.

Bill Silver