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  1. #1
    CB750 Member tdskip's Avatar
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    Rebuilt the carbs - still flooding badly

    Hi guys - my 1980 CB750F sat for about 12 years before I bought it. It's a pretty original bike including the factory carbs. I pulled them off to rebuild them, but just for grins actually ran the bike briefly off them and it ran surprisingly well but #2 carb was overflowing like crazy.

    I rebuilt them using the kits from 4into1 after checking the floats were still floated and didn't have any holes in them. I used new float valves (that shut off and installed them properly.

    Got them back on the bike and it fired right away and ran great but then I got pretty bad overflow from #1 and #2 this time. I did NOT soak the bodies in cleaner, and am wondering if there is crud where the float valve seats are that is preventing them from flooding. Any thoughts on that or other ideas?

    Thanks!

  2. #2
    CB750 Guru amc49's Avatar
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    New needle quality has been a huge problem with most carb kits and even more if your gas now has ethanol in it. The ethanol swells the rubber tip to not seal right. The Chinese parts do not often use an ethanol resistant rubber. Needle seat corrosion can pit the seats too.

    Beyond that, it is up to the person doing the rebuild work to guarantee the openness of any fuel passages rather than guessing at it. Don't discount cracking in the bowl overflow tubes either if you are somewhere where it can freeze commonly, moisture then freezes in the tubes to crack them.

    If you can see the superfine red rust in bottom of bowls then get set for some fun, virtually no filter on the planet can catch all of it, it goes right past the OEM tube filter in the petcock. From tank rusting due to ethanol. Ethanol is the enemy of all classic motorcycles, they vent to atmosphere way too much and keeping water out of the fuel is next to impossible. That red rust dust will make the needles continue to leak when it gets on the seats.

  3. #3
    CB750 Member tdskip's Avatar
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    @amc49 - thanks for the detailed response. I never thought to check the tubes, let me get those inspected.

    Seems unlikely that the rubber would go bad within hours, no?

  4. #4
    CB750 Guru amc49's Avatar
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    I've seen needles swell out of shape in 24 hours, the wrong rubber there will not tolerate ethanol at all. Most people never grasp that even normal good fuel with no ethanol in it will swell rubber in minutes but that small amount is expected and within tolerance of the fitting of parts. I've rebuilt carbs with relatively new o-rings that were OEM but bike sat for a little bit, the rings still were soft but leaked at the carb connecting tubes to drip everywhere. Turned off gas and let the dripping fall off to zero, next day petcock turned back on and zero leaks, the swelling had swollen the o-rings back to proper shape and I then ran the exact same parts for years. You learn to use things like that to your advantage. It is the amount of VOC in the fuel that determines how much rubber swells, and why they say not to use like carb cleaner on carb rubber parts, the extra xylene and acetone in the cleaners will warp them to where you don't even recognize them. It seems to ruin them but if you don't freak out and handle the parts roughly (they tend to tear super easy in that state) you can often let the parts sit for a couple days and then they shrink back into original shape and density to then use over again, I've done it way more than once.

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