I might have a problem with the float height on my CB400T from '78.

It seems to be running rough at a constant speed, however when i accelerate it'll go away until constant speed again. It's like the right cylinder is not getting enough fuel as it comes on and off.

Why i think it might be the float height is because whenever i shut off the fuel, the problem seems to go away and the engine runs much smoother and the constant speed problem seems to be gone. When i then turn the fuel back on the problem is there again.

Does this sound like a float height problem, and if this is the case would the float height be too high or too low? Thanks

  • Welcome to the site. The best way to ask a question is to just describe what problem you are having without including speculative diagnosis, as it can cause misleading answers. Describe all symptoms in detail and what you have tried to resolve it. What you have described could be a number of things. When was the last time the carbs were cleaned? Has this bike been sitting with fuel in it for a long time? How do you shut off the fuel and the problem goes away? Doesn't the engine quit with fuel shut off? – CharlieRB Jun 12 '17 at 13:25
  • "Does this sound like a float height problem" Yes! or a faulty needle and seat. – Moab Jun 19 '17 at 0:43

I believe this is a mismatch in your floats as the right cylinder works fine when the petrol is off would recon you to get it checked with a authorized mechanic who knows what he is doing also would be a better idea to get the carbs swapped and checked.


Assuming you can get the bike to reproduce the issue in neutral you can confirm it's the right cylinder by putting your hands over the exhaust pipes and feeling the exhaust pulses when it's doing it. If you have a friction screw on the twist grip you can use it to hold the throttle at a steady speed.

You could also have some crud or something preventing the float needle from sealing. You would probably notice the carburetor overflowing on the problem side at low rpm and idle, although a vacuum operated pet cock might mask the problem when the engine isn't running.

You can drain each float bowl into a measuring cup and see if there's a big discrepancy in float height.

It looks like you have cv carbs on your bike. It's possible that the slides aren't synchronized with each other.

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