Vacuum Petcocks Not Shutting Off

[Guest]

With the factory vacuum petcocks on some of the earlier GS's, there is a design 'Feature' that causes the petcock to NOT shut off when removing the fuel line from the tank. This occurs when you turn the petcock from PRIME to RUN after the bike has been shut-off.
Here's what's in the factory manual:

When the engine is not running and the valve in the on position, the fuel valve is kept in the closed position by applying pressure utilizing a spring so no fuel will flow to the carburetors.
When the engine is engaged, a negative pressure is generated in the diaphragm chamber "C" through the vacuum (negative pressure) pipe which is connected to the carburetors, and builds up a negative pressure which is higher than the spring pressure so the diaphragm is forced to open the fuel valve and thus allow the fuel to flow to the carburetors.
On the other hand, setting the valve in the ON position keeps the air return orifice open.
Negative pressure does not accumulate on the diaphragm at the time the engine is stopping, and then the spring pressure actuates the diaphragm to move back into its original position and closes the fuel valve.
However, setting the valve in PRI position causes the air return orifice to close, resulting in negative pressure in the chamber "C" under the diaphragm. This negative pressure doesn't allow the fuel valve to close and therefore fuel keeps flowing to the carburetors.

In other words, you need the vacuum to reset the petcock after moving the lever from Prime to Run...

[buddha]

Great find! I'm sure there are about 100 threads in here involving leaking petcocks and needle valves that can finally find closure with this info.

Good to know!

[Griffin]

Hmmmm.....I wonder how many "leaking" petcocks have been changed out because of this?

I'm wondering now about my GS1000G's petcock. It's "leaked" once in the past month while sitting in the garage. All the rest of the time, it's been tight. I was thinking about replacing it. I wonder if I'd fiddled with it after I turned it off and caused it to keep passing fuel downstream....

[SqDancerLynn1]

Based on that, when you turn it to on the air vent opens closing the petcock. So it doesn't matter if the bike is running or not when the valve is turned

[Dave_17954]

With the factory vacuum petcocks on some of the earlier GS's, there is a design 'Feature' that causes the petcock to NOT shut off when removing the fuel line from the tank. This occurs when you turn the petcock from PRIME to RUN after the bike has been shut-off.
Here's what's in the factory manual:

Shouldn't the petcock be in the "run" position when the bike is on/running? Only time I turn mine to prime and only for a few seconds is when it has set for awhile or when its really cold out to let some fuel run through to make it easier to start. other than that mines on the run position almost all the time. only time mine leaked in the run position was when my diaphragm went bad and it didnt hold vacuum.

[Matchless]

This should be able to be proved easily enough. Pull the pipes off and if fuel leaks out of the fuel line, just give a quick suck on the vacuum line and see if the fuel flows fully and then shuts off completely.
If anyone proves this please post.
Thanks.

[Billy Ricks]

One thing that causes fuel to flow at any position is when they are disassembled the push piece that acts on the diaphram in the prime position might get adjusted too long. When that happens it keeps pressure on the diaphram. Conversely, if that piece is adjusted too short the prime function doesn't work at all. The piece I'm talking about is a flat rod with about a half loop of spring built into its base. It's right behind the barrel valve that controls the run and reserve positions. Sometimes it takes a coupleof attempts to get it adjusted right. Sucking on the fuel line will let you know when you have it right. Of course that's done with the petcock removed from the bike.

[Griffin]

One thing that causes fuel to flow at any position is when they are disassembled the push piece that acts on the diaphram in the prime position might get adjusted too long. When that happens it keeps pressure on the diaphram. Conversely, if that piece is adjusted too short the prime function doesn't work at all. The piece I'm talking about is a flat rod with about a half loop of spring built into its base. It's right behind the barrel valve that controls the run and reserve positions. Sometimes it takes a coupleof attempts to get it adjusted right. Sucking on the fuel line will let you know when you have it right. Of course that's done with the petcock removed from the bike.

Thought a bit of editing would be a good idea......

[Billy Ricks]

thought a bit of editing would be a good idea......

excellent!!!

[49er]

Leave them in the run position. As stated earlier, turn to prime in very cold conditions or when the bike has sat for a period since it's last run, just to start it. Then switch straight back to on/run.

After re-kiting mine, I re-checked it's vacuum operation with fuel in the tank as well.