Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Gary400/4 on October 06, 2026, 11:23:12 AM
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Hi My honda CB400/4 was running good until recently when I Investigated the carburettors float heights because of constant flooding I suspect the floats have become impregnated with ethanol but I cannot locate any ethanol resistant floats, has any one else
had a similar issue
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Floats can become porous after 50+ years but I've never known all 4 to go, maybe just 1. The floats aren't affected by Ethanol, are you using E5?
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Just in case you disappear down the same rabbit hole that I did,A few years ago, I had similar problems. Suspecting float issues l cleaned the seats and fuel galleries etc, I set the float heights carefully, assembled and it still overflowed.
Ordered a float needle set & seats with some rubbish off fleabay. Didn't know about this forum then. :-[ I had a spare set of carbs from my own project bike and replaced the floats and needles and it still overflowed! I was getting pretty desperate by now, so I did some research and found a site from the USA (I can't find the link sorry) where a chap, was over hauling his own carbs. He mentioned checking overflow tube in the bowl. Knowing damn well it couldn't possibly be that, I checked it anyway and guess what, it was the overflow tube! I placed the bowl on top of my bench and supported it, poured petrol in it below the level of the tube and petrol poured out of the overflow. I have read that some people suggest that the brass tubes can crack through age and vibration. Someone else suggested it maybe cleaning using an ultrasonic bath, but it's not entirely unknown. Maybe worth a check if you haven't already.
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They crack when water gets in and freezes
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That would make sense
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The only time that I had a persistent puddle of petrol on the floor, the float was twisted causing it to get stuck somewhere between full and half full. Lay the flat bottoms of the floats on a flat surface and see if they both sit level.
The other possibility is crud from the tank or debris flaking off on the inside of the (if its old) fuel pipe getting lodged in the float needles. Pay attention to any sediment in the float bowl when you take it off.
When filling rebuilt carbs for the first time, a gentle tap on the offending bowl with a screwdriver handle can sometimes get the floats floating again.
Regards
Dave