Overflowing Carbs

[215Guy]

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.

[Re-run]

Most likely the needle valve or seat is messed up.
The needle valve can stop sealing properly from wear. But, before replacing those, the seats get buildup on them that also stops sealing.

Some times a simple cleaning with a q-tip and carb cleaner will do it. Other times, some fine steel wool and a q-tip is needed. A brass bristle brush that fits can do the job too. Just go easy on it and take your time.

The other possibility is that the floats have holes in them and so do not float, or they are set too high and not shutting gas off.

[215Guy]

Thanks for the help.  I actually already replaced the needle valve and cleaned the seats pretty well, or at least I thought I did.  I also at one point let each of the floats soak in water for a while to make sure they didnt have holes in them, and all seemed good.  And with my model and the plastic floats, apparently there is no height adjustment.

[Re-run]

Could be cheap needles. I had gotten some that were all metal on the tips and they did not seal as well as rubber tipped needles. They were really inexpensive, so that probably explains it.

[icerigger]

Check for cracks in the overflow tubes. Sometimes they can be hard to see.