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  1. #1
    CB750 Member 215Guy's Avatar
    Join Date
    Nov 2013

    Overflowing Carbs

    Working on a 1981 CB750 Custom. Got the bike recently and a couple of the carbs would overflow with the gas on. They would stop after tapping them, but would start again intermittently. I was able to run the bike briefly but then took the carbs apart to clean them. Replaced the gaskets and jets from a rebuild kit. Got them back on the bike got it started, everything sounded great. However carbs 3 and 4 seemed to be leaking gas from the seal of the float bowl. I took them back apart, replaced all the bolts holding the bowls on with allen heads so that I could tighten them up without worrrying so much about stripping them. I also inspected the gaskets and decided that the old ones, even though they were a little worn, were still better than the low quality ones that came with the rebuild kit. put the old ones back in, no leaking from the seals anymore. However, the overflows are back to spitting gas, and it seems like its actually gotten a lot worse. The floats seemed like they were in good shape, and with the carbs apart seem to move freely. But especially in carb #1, there is definitely nothing stopping the flow of gas into the bowl. It sounds like its just a cleaning issue, but I just did that, so before doing the same thing over again, I wanted to see if anyone had any other thoughts on what it could be. Thanks for any help.

  2. #2
    Old Bike Nerd grepper's Avatar
    Join Date
    Oct 2012
    Lowell, MI
    Definitely sounds like a float valve problem. Could be a lot of things, floats binding, valve worn out, seat warn out, crud in there keeping it for sealing up.
    Drive fast and take chances

  3. #3
    Or a faulty set float level.
    Everybody has a plan. Until they get a bee in their helmet.

  4. #4
    CB750 Member hondsx's Avatar
    Join Date
    May 2013
    had help cleaning my carbs, 3 different shops and friend builds street hot rods (eats, sleeps, dreams carbs and engines)
    gunk from old gas cleared, jets "rodded", new needles, new gaskets, re-set float heights, and yes - tapping (gently most of the time) each float bowl

    still have problem "occasionally" too

    dawned on me, will go back and check, but fuel hose (very, very old N cracked) (1" - 1 1/2" BETWEEN carbs) could be "intermittently" deteriorating and breaking down off minute particles ("microscopic")(fuel gunk) at the oddest of times.

    will advise (try to... ) once they are replaced.

    In the mean time, re-check float levels and ensure "100% free play" plus blow forced air thru all jets, to get rid of "minute" gunk, check hoses any unequal pressure (air seepage) would change fuel pressure to that carb.

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