Motorcycle Repair: Honda cb750c 1981, honda cb750 custom, vacuum leaks

Question
Hello,

I've recently bought an '81 Honda CB750 custom. I knew it was going to be trouble when I bought it. (I like a challenge) I've managed to start her up after thoroughly cleaning all the carbs. The air cut-off valves are a bit eaten away, but still seal. and i'm unable to locate the accelarator pump. (the diaphragms are EXTREMELY expensive, $25 a piece)

The bike isn't able to idle, the revs jump between 2000 and 3000, any lower and it will stall. It will only start with choke on full, and it will also backfire occasionally. When I turn the throttle, the rev immidiatly drop to the point of stalling.

I think this is a combination of problems. The air cut-off valves and accelarator pump will cause the engine to stall when I turn the throttle, however I can't figure out why the bike won't idle.

I've checked for vacuum leaks, and adjusted the pilot screw (1 7/8th turn open).

I hope you can help me with this problem.

Answer
Hi Huibert,
It sounds like a tough one, if the air cut-off valves are not up to spec that will be a problem. You have to consider other factors also. Are there any other minor issues you know of; air filter, synchronization,  float levels all correct? Is the bike able to pull itself with you riding it, running at 3000 rpm and above? Do you know the correct pilot screw setting?
Regards,
Terry.