Motorcycle Repair: 1991 honda cb750 nighthawk running problem, honda cb750 nighthawk, oem honda parts

Question
Bill,

I just purchased this bike recently with 44k on the odometer. The other day the bike started acting up. When it is cold it will fire up and run great but as it starts to warm up and the choke is kept steady at the halfway point, the bike rpms slowly start to drop to the point of the bike will die and not restart until it cools down.

The drive-ability symptoms are as if it is misfiring on one cylinder but I have checked the ignition system (coils, plugs, wires) and it all checks out.

This problem is also somewhat intermittent and is really annoying.
Any thoughts?

-William

Answer
William,  at 44k and 21 years of age, I would start by draining the fuel tank and replacing the fuel screen inside the tank, above the petcock.

http://www.cmsnl.com/honda-cb750-nighthawk-1991-usa_model484/partslist/F++13.htm  #4

In the same area, there is a small screen inside the petcock fuel bowl. The "misfiring" on one cylinder is probably related to a failed diaphragm in the automatic shut-off valve, which is part of the petcock assembly. When the diaphragm fails, the vacuum line to the carburetor becomes an air leak, leaning out that cylinder and at the same time shutting down the fuel supply to the carburetors.  #6 on that illustration.  There are aftermarket repair kits for the shut off diaphragms that may be cheaper than OEM Honda parts.

If your battery is fully charged and charging system is keeping it up, then the probable cause is the fuel delivery items mentioned above.

Bill Silver