GS750 1977 Fuel flow

[ybot1966]

I have a GS750 1977 with a vacuum petcock with prime setting as well as on and reserve , petrol leaks from the left hand carb into the air cone after running the engine but stops randomly sometimes. My question is, if the vacuum fuel system is working properly should it only fill the carbs once even if the carbs are stuck (which can be emptied underneath) so is it correct that if fuel is flowing continuously through the carbs its a problem with the petcock. if a carb is stuck open does this trigger the vacuum flow of fuel regardless of the engine running?

Thanks

[Curly]

If the engine is not running, and fuel is pouring out of a carb's drain plug, I would think that the problem is with that carb's float. Which either has a hole and is weighted down, or the needle the float controls is stuck. If it's directly under the carb which is fed by the petcock, it could be that it's not closing when the engine's vacuum goes away. It should be more fuel than if just a float needle was bad. It's a cheap rebuild kit, so you might as well.

[Suzuki_Don]

Faulty petcock and the carbs need to be cleaned and the float valves and seats checked and replaced if necessary.

[ybot1966]

Hi thanks for the info
The carbs have K&N type air filters on each, petrol is dripping out of the left hand one, what i wanted to clarify was even if the floats and carbs are stuck and open presumably only a carbs worth of fuel would leak not a tankful, which suggest my petcock is stuck?

cheers

Toby

[Coady]

Hi thanks for the info
The carbs have K&N type air filters on each, petrol is dripping out of the left hand one, what i wanted to clarify was even if the floats and carbs are stuck and open presumably only a carbs worth of fuel would leak not a tankful, which suggest my petcock is stuck?

cheers

Toby

True. Plus whatever's left in the fuel line.

Pull the fuel line off the petcock and you'll know whats happening there...

[ybot1966]

thanks it seems an intermitnt problem made worse after riding ill try a petcock rebuild probably the diaphram

[Steve]

If you have fuel flowing out a carb throat when the bike is not running, you have THREE problems.

1) Your overflow tube is plugged. It should handle excess fuel before it gets to the level of the throat.

2) Your float valve is stuck (or mis-adjusted). It should prevent fuel from getting above the proper level in the bowl.

3) Your petcock is stuck. It should stop ALL flow when the engine is stopped.

CV-type carbs in the newer bikes don't have the overflow tubes, so they only need TWO of these (#2 & #3) to get fuel to come out the throats.


Back to one of your original questions:
"if a carb is stuck open does this trigger the vacuum flow of fuel regardless of the engine running?"
Only a running engine is going to produce vacuum to open the petcock. Flowing fuel can not do that.

.

[ybot1966]

Thanks Steve
there doesnt seem to be an overflow on the carbs just a screw at the bottom that allows fuel to be drained. i think the main culpret? is an intermitent faulty petcock as the petrol just keeps coming regardless of whether its on the reserve or on position, the prime position just speeds up the flow as it should.

Toby

[GS ace]

Fuel Leak

The problem is most likely being caused by a bad diaphragm in the petcock. Seepage can occur without vacuum. Gas could find its way into the crankcase, so it's important to check the oil. It has happened to me on my 1100E.

[ybot1966]

Thanks
i dont want it to hydraulock! so will drain the oil if it smells of petrol
In addition, does anyone know of petcock kits from the Uk I see loads on ebay from USA?

Toby

[Curly]

Thanks Steve
there doesnt seem to be an overflow on the carbs just a screw at the bottom that allows fuel to be drained.

So you're just checking the drain plugs every so often? WTF would you do that? How did you even find this issue? Did it ever come out of either the throat of the carb or the over fill line?

[ybot1966]

I cant find an over flow line from the carbs, the petrol was dripping out of the back of a K&N style air filter on the left hand carb under the petcock, i unscrewed the drain plug at the carb bowl to see if the leak stops after being drained, but it just keeps coming so deduce ins a bad diaphram? in the petcock that seems to be sparodic

Toby

[Coady]

The overflow is the brass tube at the centre bottom of the float bowl and should have hoses running to the rear of the bike dropping down behind the swingarm bolt.

You should be able to drain a float bowl like you did and then fuel flow stop completely, otherwise stuffed petcock. Could be diaphragm or the rubber behind the tap tab perished so bikes always acts like its in prime.

[BassCliff]

Greetings and Salutations!!

Hi Mr. ybot1966,

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Thank you for your indulgence,

BassCliff

[Steve]

there doesnt seem to be an overflow on the carbs just a screw at the bottom that allows fuel to be drained.

..., the prime position just speeds up the flow as it should.

There should be a fitting on the bottom of your carbs where a rubber hose attaches. The hose is there to direct any excess fuel away from the engine and the rear tire.

And the 'prime' position does not "speed up the flow" of fuel, it just bypasses the need to apply vacuum to the petcock to have fuel flow.


This is the parts diagram for the carbs. There are only two of the four overflow tubes pictured, but they are #44.