Motorcycle Repair: Honda nighthawk not firing on one cylinder, honda 750 nighthawk, petcock valve

Question
Hi Wayne.
I have a 1992 Honda 750 nighthawk that suddenly started running rough.
Actually, it idles fine but seems to not kick in all 4 cylinders when I
accelerate. I replaced all plugs (none fouled), no improvement. I have now
removed the carbs for cleaning and will rebuild while I'm at it. But I'm
wondering if the carbs are the most likely culprit, since this the rough
running came on overnight. Running great when I parked it, poorly next day.
The fact that the plugs were not fouled directed me away from electrical, but
should I suspect coils? Other thoughts?   Many thanks for any advice.

Answer
Peter,

-sometimes the vacuum line to the petcock
can suck air or possibly fuel through pinholes
in the petcock valve rubber seals.

-There may be a filter screen in the tank
part of the petcock, check for good fuel supply.

-The petcock is opened by vacuum from the engine
which is lower at higher speeds which makes it harder
to open the petcock for fuel if the petcock has
any faults.

-You may want to check the compression
to see if the cylinders are near equal PSI.

-you could hook up a timing light
and see how the spark is at different speeds
on different cylinders.

-if the spark looks okay, try draining
the float bowls, check the amount of fuel
in each bowl.

-Check the spark plug caps (5k ohms).

-Make sure the main jets and the needle jet
tube under the main are physically clean.

-always have a fully charged battery and check
the battery connections and grounds to frame for
any corrosion.