QuestionI have a 1981 CB900. When i start the bike all 4 carbs will start overflowing gas from the vaccum ports. It was running fine and just started doing this one day. This bike had sat for a couple years before i got it. I have taken the carbs off and put a spare set on and they do the same thing. Does this bike have a diaphragm in the petcock and could this be my problem?
AnswerDale, I am not sure I understand about "vacuum ports" reference. Are you talking about the air intakes or the overflow tubes or ?
Is it a CB900F or CB900C?
http://www.cmsnl.com/honda-cb900f-900-super-sport-1981-usa_model1023/partslist/E
is the automatic fuel control. There is a diaphragm in it that can fail, which generally gives an air leak and lack of fuel supply, not exactly what you are describing, from what I can tell in your description.
Make sure that the fuel tank is venting properly.
When the carbs run low on fuel the floats can hang down and cause the float valve needles to hang open. When the carbs are overflowing, shut off the gas at the petcock, run the bike and shake it back and forth and/or tap on the carb bodies with the plastic end of a screwdriver or something that doesn't damage the carb bodies, but gives a shock to the carb to reseat the float needle.
Bill Silver
