I have an '80 Honda CB750 which I am trying to improve. I’ve cleaned the carbs, replaced the float needles, etc but i’ve got one carb that just won’t stop overflowing, as if he float were stuck. I can’t detect anything wrong with the floats, or the rest of the carbs for that matter, but it just keeps overflowing. Anyone out there know the trick to getting them right?
Thanks!
Hammer
Its possible that the float has developed a pin hole in it, so it is partly filled with fuel and not lifting enough to close the inlet.
Being a Honda, you will probably find its made of white plastic so you should be able to see if there is fuel inside the float. Sometimes the float becomes slightly deformed with age and it catches instead of rising properly, you might only see this if you compare it with a known good float.
Next, check the float valve for grooves and nicks, even the smallest damage will stop it sealing correctly.
There is a measurement you can make on the float level but I don’t have a manual for the CB 750. Suffice to say that it should be replaced it it does not close properly.
The float, pivot pins, and float valves can wear, but this should happen to all 4 carbs at once, anyway, if you are going to replace one set, it makes more sense to replace all 4 in one go.
These bikes had a vacuum diapragm connected to the inlet manifold, so that the carbs are completely shut off from the tank when the engine is stopped.
Those diaphragms are notorious for developing pin holes in them as the rubber ages, I don’t know if this would overflow the carbs, on my CBX 550 it just prevented fuel going into the carb but you might take a look.
You could replace all the suspect bits and still end up with leaks if the float valve bed is worn, and this is not a serviceable item.
The carbs should be good for over 100k miles though so its up to you to gauge wether this is likely to be the case
Knowing he exotic price of Honda spares, you should price up the floats, float valve and float pivot pins, 4 of each,(plus the obligatory seal kit) and compare this to the price of a set of secondhand carbs from a low mileage wreck in a breakers yard.
Yes, the floats are made of plastic, and I can’t find any fuel in them. I just replaced the needles, no help. I do have the manual and I know the setting, but there is no way to change it, as this float setup is non-adjustable.
As to the vacuum diaphragm, this year didn’t have one. I wish it did, because forgetting to close the petcock results in gas all over the place, as you might guess.
As replacement for all this adds up to quite a sum (would you believe $50 ea for just floats?!), I am hoping to find a better solution. With luck, I just have an undetected bit of grit in the seat. I never cleaned it with anything more aggressive than a Q-tip.
Thanks for the words…
Blast the carbs out with compressed air, even the service manual recommends this.