View Full Version : Gas flow issue driving me insane.

07-11-2011, 12:35 PM
I have an issue with a friends bike he wanted me to look at. It is a 78 cb400a that I just cant seem to get to run right.I have had the carbs apart and maticulously cleaned and readjusted float level and recalibrated 4 times with no luck at all. The bike does not want fuel to flow for some reason and I have removed the in tank filter and cleaned the vent in the cap. You can hook up the gas line to the carb and the gas will not flow but the minute you remove it flows like crazy. I also have taken the bowl off while on the bike and moved the float up and down and it will flow just fine when needed but the minute I put the bowl back on it stops flowing what is up with this. Is there some kind of vent on the carb I am missing but it seems that when I put the line on the carb it has no air to draw the fuel to fill the bowl so it can fill the other bowl so I can proceed. I am not one for throwing in the towel but this one has me stumped to that point. I have worked on alot of bikes and this one is pissing me off. I have never had so much trouble with a bike than this one is it my WHITE WHALE. Any Answers for me that I havent allready tried would be helpful Thanks

Gas Man
07-11-2011, 08:06 PM
Does it have a vacuum activated petcock?

Did you check the fuel line valve that hooks to the float?

What type/brand of carb are we dealing with here?

07-12-2011, 12:46 AM
No it does not have a vacuum activated petcock. These are Keihin vb carbs. There is no fuel line valve hooked to the float. Thats what is puzzleing me I have never seen this happen before I am loss for words and frustrated.

07-12-2011, 01:44 AM
so i'd guess you left the gas cap open and it still does it?
theres a vent port or ports plugged. either pull off the floats and shoot air in the vents or pull the carbs and shoot carb cleaner in them and air. i had the same problem and i know it's enough to drive someone crazy lol.

what did you clean the carbs with? i've found the carb dip don't always do the job.

07-12-2011, 10:29 AM
I did leave the gas cap open and it still would not flow.I cleaned every passage in the carbs where is the vent port cause the last time I ultra sonic cleaned them in my special brew which works everytime. Its just wierd I can take the bowl off and turn on the gas and drop the float and it flows gas like normal and push up and stops the flow like its suppose to. But as soon as I put that bowl back on it like seals it up like air tight and it will not allow gas to enter.

07-12-2011, 10:56 AM
I know it will be hard to tell but maybe the float bowls are pushing the float up some how and shutting it off. If a vent was plugged you would still get a little gas in the carbs until the pressure built up.

07-12-2011, 12:05 PM
Well the floats are set to specs and they hang down cause Ican see them as I put the bowls back on. I did clean both overflow vent tubes and they are clear and flow air respectfully as they should until the bowl is full. The funny thing is I can squeeze the gas line and I can here it squirt into the bowl somewhat.