unusual carb leakage: '76 cb550 four

[dyelawn]

Happy to report my '82 CB750 is running strong.  However, I'm working on restoring a 1976 CB550 (K?) four cylinder with my girlfriend, and we're having carb issues.  The bike ran great for about 1 month but now out of nowhere the #2 carb is leaking gas out of the overflow tube.  We ripped the carbs off and opened the float bowl but everything seems to pass inspection.  The floats move freely and the needle seems to press the valve closed easily when moved to the up position.  I even took the #1 bowl off and compared: the two float mechanisms seem identical.  What's more, when I put everything back together and ran gas into the carbs (still out of the bike), it seems like the #3 and #4 are leaking through the overflow tubes as well, now.

What I'm really wondering from you wise old-timers is if there is something else I'm missing, possibly some sort of spring or screw that governs gas flow and shutoff into float bowls for all the carbs at once.  What are the chances that three floats failed simultaneously after working perfectly?  This seems too unlikely to me.  What would you guys check next if you had carbs leaking but the floats/needles/pins/valves seemed fine and free of debris?  What could have caused a bunch of them to fail all at once?  Maybe I was just holding them slanted when I tested them yesterday, and that's why #3 and #4 are started overflowing?

All we did was take the tank off to switch out throttle cables.  Then this started happening.  Obviously I thought it could be the new cables causing the problem, but even with them totally unhooked my carbs are still leaking.  HELP!  These are the older style, thinner/taller carbs on the SOHC Hondas.  Thanks all.

-Dylan

[TOOLS1]

I had this same thing happen on my friends 76 CB 550 last year. I tried polishing the needle seats with a Q-tip, and chrome polish, I tried adjusting the floats, but ended up replacing the floats. This Ethanol gas kills the plastic floats.
TOOLS  

[dyelawn]

Do you think gas has gotten inside the floats causing them to "sink", or essentially not rise properly even when the float bowl is filling with gas?  When I drained and opened the float bowls, the floats still seemed dry, light, moveable, etc.

At any rate, did replacing the floats fix the problem?

[TOOLS1]

Yes replacing the floats cured the problem on his CB 550.
TOOLS

[dyelawn]

Hmm…

But I'm right in assuming the individual floats in each individual carb are the only mechanisms which control the flow of gas into the carbs, right?  Just seems like quite a coincidence that multiple floats could have failed all at once when all I did was replace the throttle cables…

Any other insight, thanks.

[TOOLS1]

There are three things that control the height of the fuel in the float bowels, and one other thing that could make them leak out the overflow. First is the needle valve. It could have a bad tip. Second is the seat. It could be dirty. Third is the float. They could be out of adjustment, or heavy from absorbing gas, or even porous from age, and ethanol gas eating them away, or both (this is what I am suspecting here.) The final thing is that the little tube inside the float bowel has a tiny crack.
TOOLS  

[dyelawn]

OK, well, I'll get to work then.  Going to try cleaning the needles and seats first and hope it's not the floats, though you very well could be right.  Q-tips and chrome polish?  Anything else recommended for cleaning?  Thanks TOOLS.