Motorcycle Repair: Honda 750 nighthawk, honda 750 nighthawk, vacuum leak

Question
Not a twin but hopefully this is a general enigne question that maybe you can
offer some advice on-!999 air-cooled Nighthawk 750four with carbs sat for
about 2 years.  Recently paid shop to refurbish and they said they went over
everything, bike is in good shape  and runs ok but seems to have a constant
"miss" or similar sound and feeling that is pretty constant from idle up
through the RPM range, the bike feels better at high rpm's than low but not
intermittent problem, this is a constant weakness.  Used digital temp gauge
and all headers are reading within a few degrees of each other, carbs were
cleaned and synched, pulling a plug wire just results in more pronounced
problem with all cylinders equally.   I beleive the problem may be in the
timing...is this possible?  My friend believes more of a fuel problem?  Any
help appreciated.

Answer
Brian, with equal temperatures on the exhaust pipes, apparently there are no major cylinder imbalances (compression or serious misfiring) present. The ignition is all electronic with an expensive control module controlling the spark timing. Certainly an automotive timing light clipped on a plug wire could verify if the CDI ignition module has some issues in the spark timing advance curve. If you had fuel feed problems, you would probably see more of a difference in the problem at different speeds.
You could check the coil output and plug wires/caps for similar resistance values. I would inspect all wiring connections, including grounds, just to be sure that there are no corrosion issues.

Again, it seems like there is one thing common, which looking at the illustrations on BikeBandit.com could indicate a petcock problem. If you have a small vacuum line coming off the petcock, next to the fuel line, then you have a vacuum operated diaphragm. The diaphragms are known to develop pinholes or tears, which cause two problems.... restricted fuel flow and a vacuum leak into the manifold. Definitely look at any and all vacuum lines, fittings and connections for damage or deterioration.
Make sure that the tank is venting properly, too. Old gasoline vapors can plug up small passages which allow the atmospheric pressures to balance in the tank.

Bill Silver