Motorcycle Repair: fast idle when warm, rich fuel mixture, diaphragm problems

Question
Hello , I have a 1982 honda 900f.When bike is cold it idles at 1200 rpm ,when engine warms up and after a couple minutes it is reving at 3500 or higher. I had a second set of rebuilt carbs put on , new air cleaner , sea foam additive in the gas, premium gas.Same problem.  Plugs are black , milage is poor. Seems to me a carb problem but I am stumped. Any ideas?

Answer
Dave,

-if the plugs are black it indicates
an overly rich fuel mixture or lack of air.

-I suspect your gastank fuel valve is allowing the engine
to suck raw fuel in the vacuum line.
If so you will need to get a rebuild kit
or new petcock valve.

-Other reasons for this could be
low compression, choke misadjusted
stuck or worn, air intake blocked,
carb slides switched,
air cutoff diaphragm problems.
Wrong jets.