Cb750C leaking gas

WhyteRaevin · 01-11-2009, 3:43 PM

I have an issue with my 1980 CB750C leaking gas from the carb overflow drains as soon as the fuel petcock is turned open. It just started doing this last summer right before I parked it for the winter. Also when the fuel petcock is shut off it won't run itself out of gas like one would expect, it just idles away happily. Anyone have any ideas as to what would be causing this symptom? I'm stumped.

MSGT-R · 01-11-2009, 3:49 PM

If you're getting flow out of the overflow tube, your needle & seat in the float bowl is not shutting the flow off. You probably have a little dirt in there to keep them from seating, or they are worn out. Time to pop the float bowl and do some cleaning.

WhyteRaevin · 01-11-2009, 3:56 PM

If thats the case then I'm severely confused. Just had the carbs professionally cleaned last spring. They shouldn't be dirty, all I do is run high quality premium gas through this bike because it is an unrestored original bike with less that 12,000 kilometers on it (thats 7500 miles for you american folks). I only rode the damn thing about 900 kilometers last summer, if that. There shouldn't be anything worn out on it either, the guy that cleaned up the carbs for me told me everything was mint mint mint.... *shrug* I dunno. Not looking forward to taking it apart myself or having to pay someone to do it for me. ****.

lanbrown · 01-11-2009, 4:03 PM

Have you checked the petcock?

MSGT-R · 01-11-2009, 4:59 PM

The petcock won't make any difference if the floats are cutting the flow off. The petcock can cause problems, but not to make the carburetors overflow.
The floats (attached to the needle & seat) shut off flow when the bowl fills up, just like a toilet tank. Something (dirt, wear, or adjustment) is preventing the flow of gas from shutting off. It's just that simple. It's not rocket science.

lanbrown · 01-11-2009, 5:17 PM

Quote:

Originally Posted by MSGT-R

The petcock won't make any difference if the floats are cutting the flow off. The petcock can cause problems, but not to make the carburetors overflow.
The floats (attached to the needle & seat) shut off flow when the bowl fills up, just like a toilet tank. Something (dirt, wear, or adjustment) is preventing the flow of gas from shutting off. It's just that simple. It's not rocket science.

yes but see below. I made one portion bold.

Quote:

Originally Posted by WhyteRaevin

I have an issue with my 1980 CB750C leaking gas from the carb overflow drains as soon as the fuel petcock is turned open. It just started doing this last summer right before I parked it for the winter. Also when the fuel petcock is shut off it won't run itself out of gas like one would expect, it just idles away happily. Anyone have any ideas as to what would be causing this symptom? I'm stumped.

The above bold selection says to me that he has a petcock problem. He could have two issues, but why not get rid of one of them first and then check the second one out. If you turn the petcock to the off position, no fuel should be going to the carbs at all.

denzee · 01-11-2009, 5:30 PM

With the petcock off the bike will run for several minutes still at idle. Maybe Wyte expects it to shut off immediately. Also if the petcock were still allowing fuel through, fuel would also continue to leak from the carbs.

Even though the carbs were just done it doesn't take much to keep the needle valve from seating properly as MSGT said. The process of cleaning everything on an old bike can cause more loose debris in the system. Was the fuel tank flushed and cleaned as well? Try to isolate the individual tubes to find which carb is leaking. Generally you can then just remove that one bowl to clear the debris in the valve.