Motorcycle Repair: stalling/low idling.

Question
1983 Honda 550 nighthawk runs great but when warmed up goes into low idle and tries to stall while in neutral.  Have to give gas quick small shots while in neutral to prevent stalling. Gas fresh and bike otherwise has great power and starts immediately. Any suggestions and thanks

Answer
Orin, I guess the first thing to do is to try to raise the idle speed with the adjustment knob. If that fixes the problem, great!  Probably won't though.  I suspect that you might have a plugged idle jet in one or two carburetors so it isn't getting fuel at idle on one or more carburetors.

Other possibilities are the petcock diaphragm beginning to fail or some vacuum leaks at the manifolds. http://www.cmsnl.com/honda-cb550sc-nighthawk-550-1983-usa_model1016/partslist/F+  #7

http://www.cmsnl.com/honda-cb550sc-nighthawk-550-1983-usa_model1016/partslist/E+
#15, #17, #35 for manifolds and the o-rings which are probably pretty flat now.

Always good to check compression first, take a look at the spark plugs to see if they are fouled or looking too lean. They should all be pretty much the same color if you are firing on all four cylinders. Check spark plug caps for about 5k ohm resistance, measured off the wires.

If you start it cold try not to use the choke so it is idling slow on the idle jets. Check the header pipes quickly and carefully by hand to see if any are cold at idle. Those would have either air leaks or plugged idle jets.

Bill Silver