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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.
 

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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. ****.
 

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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.
 

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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.

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.
 

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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.
 

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Had the same problem. Is your gas line still hooked up to the automatic pet cock? If it has been bypassed (a lot of shops do this because the parts are not available to rebuild the auto petcock) then the line now runs straight from the manual petcock to the carbs. They will have capped the vacuum line with a rubber cap. If that cap cracks, or comes off, the overflow at the bottom of the carbs will leak when the bike is turned off. Recap that vacuum nozzle and this should stop the leaking. The vacuum is located between the 1st and 2nd carbs.
 

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I have the same issue on my cb900c but all of my 4 carbs are overflowing and I did also bypassed the automatic petcock, i dont have the gas tank connected i am feeding the gas with a bottle straight to the gas line and i also have the vaccum nozzle caped off but my motorcycle wont start, however i spray some gas on the carbs and i got started for 15 seconds max i was wondering if you guys could help me out with this issue this is my first build and i want to start riding but i just havent been able to get it started.
 

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I have the same issue on my cb900c but all of my 4 carbs are overflowing and I did also bypassed the automatic petcock, i dont have the gas tank connected i am feeding the gas with a bottle straight to the gas line and i also have the vaccum nozzle caped off but my motorcycle wont start, however i spray some gas on the carbs and i got started for 15 seconds max i was wondering if you guys could help me out with this issue this is my first build and i want to start riding but i just havent been able to get it started.
Have you had the carbs off and given them a rebuild? Overflowing gas out of them is a tell tale sign the float needles aren't seating. If the float needles aren't seating fuel will continue to flow out the carb vent, or will drain into the intake tract. This will happen even with a simple gravity feed as you described with the fuel bottle.

It would seem you need to remove them, disassemble, clean, reassemble. It would be wise to get rebuild kits (gaskets, or-rings, floats, float needles) prior to the disassembly.
 

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I did removed them and also cleaned them throughly I did replace gaskets and o rings however the floats seemed to be in good condition and the needles as well but I didn't change them you think that the needle is just the one that needs replacement then ?
 

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I did removed them and also cleaned them throughly I did replace gaskets and o rings however the floats seemed to be in good condition and the needles as well but I didn't change them you think that the needle is just the one that needs replacement then ?
Was there any leaking before the rebuild? You may just have a little adjusting to do.
 
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