1980 GS 750E fuel shut off question

[pdqford]

Sorry to keep bothering you good folks here.
I'm chased myself up a side ally from my main thread.

I have a GS 750E with the vacuum fuel shut off.
I'm trying o verify I have no vacuum leaks as I'm having trouble setting up my carbs (another long thread).

I pulled the petcock off to do a visual on the diaphram.
Never did understand how the prime function worked.
So I stuck a screwdriver in the slot and rotated the slot and expected to the pintle retract.
It didn't move.
Shouldn't it?

See attached.

P.S. I didn't want to pull the diaphragm off as it is glued down to the petcock.

[tom203]

Do yourself a favor and stop messing withthe petcock- just get a replacement and find other stuff to mess with regarding fuel delivery to carbs. This guy sells them...note centerline bolt spacing. Others sell them too

http://www.georgefixs.com/1980-gs550-gs750-gs850-gs1000-gs1100-petcock/

[pdqford]

Thank You for the web site. I’ll refer to it if my petcock gives me trouble.

My petcock delivers fuel to the carbs,
and now I have verified that it doesn’t pass any fuel to the vacuum side of the diaphragm.
And my vacuum pump shows it holds a vacuum.

I’m just curious how the prime function works (without having to destroy the diaphragm to see what rotating the slot does to invoke the prime function).

Sooo, if anyone knows......

[Steve]

I have not dismantled one of the "dreaded '80 petcocks" like yours to see the exact mechanism, but I can tell you how to use the PRIme function.

With the slot in the horizontal position ( -- ), you are in RUN mode. Starting the engine will apply vacuum to one side of the diaphragm, pulling against a spring, to move the 'plug' away from the port, allowing fuel to flow.

In all the other GS petcocks, moving the lever to the PRIme position will mechanically push the plug away from the port. However, your '80 version requires moving the slot to the vertical position ( | ), then applying one 'puff' of vacuum. This can be done by removing the vacuum hose and sucking on it (rather awkward to do) or by pushing the starter button so the crank turns at least two full revolutions, ensuring that the #2 piston pulls one 'puff' of vacuum through the carb. Once it has had the one 'puff' of vacuum applied, the petcock will remain open and fuel will flow until the selector is moved back to the horizontal position or you run out of fuel. I have no idea what the actual mechanism is that keeps the petcock open.

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[pdqford]

Whisperer, many thanks for that description of the ‘80 petcock. So easy to follow, too.
Looks like mine works exactly as you described.

The only embarrassing part is that I haven’t been using it correctly for the last thirty five years!

[Steve]

In that case, consider yourself LUCKY that the carbs have been holding back the fuel (as they should).

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