Motorcycle Repair: cb900 exhaust missfire, nylon mesh, honda models

Question
Bill, my 1980 cb900c smokes for a few minutes when startup than clears up.Also there is a missfire in the exhaust at all speeds.Plus it loses power above 60 mph.The bike sat for a long time [years]Than restored.Carbs have been rebuilt with new parts. compression is 120,125,120,130.what do you think? Thanks for your reply, Gary

Answer
Gary, Honda thinks the compression should be 170 psi! If you checked it with carbs on/throttle closed, it is harder to get the higher numbers. Even with the throttle open, the slides are in the way of airflow. Still your results seem low, so maybe the gauge is off or your motor has low compression due to 1) worn out/unseated rings or 2) the cam(s) are out of time. With readings that even and that low, it seems more like cam timing than anything.

Pull the valve cover and check the marks on the cams at the T mark on the flywheel/stator. There should be marks on the camsprockets that will line up with the surface of the cylinder head, usually. If one or both cams are out of time, then you have your answer.

Some Honda models, starting with 1980 have vacuum-operated fuel control valves. When the control diaphragm fails, you get an air leak and a shut down of the fuel flow. This assumes that the petcock screen inside the tank is clean and the fuel flow through the petcock is not restricted. The screen on the petcock is nylon mesh and easily plugged up.

I did check the illustrations at www.cmsnl.com, just now, and they indicate that the automatic fuel valve is a '81-'82 item, so that may not be relevant to your situation. Automatic fuel valves usually take the vacuum signal from the #2 carburetor, as I recall.

Startup smoke is probably coming from old valve stem seals, leaking some oil into the cylinders when the motor is shut off. You may notice oil smoke at a prolonged idle, which is also an indication of bad stem seals. Could also be present if the rings were never seated, after an overhaul, or there was a rust buildup on the cylinder walls because of extended storage which keeps the rings from seating properly. Sometimes the rings will stick in the ring land grooves during long storage.

Any air leaks in the exhaust system will cause backfiring, but generally at decelleration, when you let off the throttle. Either the motor is leaning out under power, because of fuel flow restrictions or the exhaust cam may be a tooth out of time.

Bill Silver