Topic: 1978 cb750K carbs

[jmac851]

I have a 1978 cb750K and I have rebuilt the carburetors. When I first install them after rebuild they worked fine. After parking the bike for a week and then turning on the fuel valve, they all leaked as if the floats were not stopping the fuel flow. I took them off and cleaned them. There was nothing inside at all. I re-installed them and they worked fine. After a few days, I turned on the gas and the same thing happened. I have done this six times with the same results.  I have 2 inline filters in the line from the tank so I assume that is not the problem. Any suggestions?

[Stev-o]

Could the floats be installed upside down?

[martin99]

Very strange that all four should leak. I had a similar problem with my PD carbs, but with two of them leaking - that is, fuel coming out of the overflow tubes - after standing for a few days. A few sharp taps on the bowls with a screwdriver handle stopped the leak.

The cure was to replace the float needle valves. I think those little rubber tips wear and harden over time and remember modern fuel with ethanol is not your friend. The tips on the new needles were certainly more supple than the old ones.

[jmac851]

I've replaced the needle valves, floats, bowl gaskets and cleaned up the seats with polishing compound. They don't leak on my test stand and then when I first install them for the first ride. It's only after sitting for about a week.  Also, they don't leak after sitting on the test stand for a week.

[brewsky]

Are you turning the petcock off when parked?

[DaveBarbier]

Is it possible that you have slow leaks in the overflow tubes that slowly empties the fuel from the bowls? Then the floats are sagging all the way down and aren't able to be raised with the force of buoyancy? I'm just thinking the needles could be getting cocked in it's guide when the float is sagging too much. Is there another tab on the float which prevents too much sagging when the bowl is dry?


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1978 Honda CB550K

[jmac851]

I found the leaks.  It was leaking from the fuel supply connector tubes.  The O-rings had dried out. Taking apart the bank to install new O-rings was very easy. The bike runs great now.  A little heavy on the throttle return spring though.