Petcock help!

[Guest]

On my 1977 gs 750, bike will run on prime and reserve but not on. Anyone every come across this?

[tom203]

On my 1977 gs 750, bike will run on prime and reserve but not on. Anyone every come across this?

Not me, I just replace my petcock when it acts funny. Prime and reserve both draw from very bottom of petcock filter area, while " on" is further up.

[rudeman]

Check your vacuum line for cracks or for failure to seal. Not sure about Reserve but Prime only requires a momentary vacuum pulse to open the valve to allow fuel to flow. The Start setting requires constant vacuum to keep the valve open. At any rate, a new one is less than $100 almost anywhere OEM parts are sold.

[hannibal]

Reserve and on should function identically. Both require constant vacuum to keep the valve open. Only difference is where it draws from. Prime does not require any vacuum and will flow with the engine off or on.

If your bike runs with the petcock on reserve but not on, you might be almost out of fuel. Try on with a full tank and let us know what happens.

[rudeman]

Prime does not require any vacuum and will flow with the engine off or on.

If your bike runs with the petcock on reserve but not on, you might be almost out of fuel. Try on with a full tank and let us know what happens.

My 1000ST and 1000EN both need a bump of vacuum to open the valve in Prime. Good point about low fuel; it was so obvious I missed it completely.

[hannibal]

I'm probably wrong. My post was based on a K&L petcock from Z1. My apologies. My stock petcock was crap and would leak through the outlet.

As for the Z1 petcocks, get the more expensive one with the bigger reserve if you decide to. I didn't get 15 miles on the other petcock's smaller reserve.

[Dub-K113]

Thanks for the replies. This happens on a full tank. I know that the reserve and on required vacuum and the prime doesn't which is why I find it funny the on is not working and the reserve is. I must have vacuum to open the reserve right? Could it be something with the vacuum diaphragm?

[tkent02]

Take the petcock out and find out. It's a very simple device.

[Dub-K113]

Roger that.

[Gorminrider]

I'm probably wrong.

No, probability is you are right. Rudeman's 1000ST and 1000EN are exceptions, apparently.

The prime position does not require any vacuum. It is intended to fill empty carb bowls and disables vacuum diaphragm by physically pushing the spring-loaded vacuum stopper off the flow OR by bypassing the stopper.
Apart from fuel tank not being filled above the reserves fuel level, if the tap works on RES but not RUN, an idea would be that the tap lever itself is not operating the mechanism correctly .

[Zukimoto]

Right Thread? Petcock issue.

Hello, I just bought a 1982 gs450l. New to bikes but I think I bought the best model Turning into more of a project than I thought but at least Im learning. So I have a petcock that leaks thru my air pods. I took it apart and stretched out the spring but it still leaks like crazy in res/on/ and prime. I am wondering if anyone has any good sites to find a replacement also if anyone has some cut-up-an-air-mattress trick or something to get me riding. Hope Im on the right thread.

[AMK]

Hello, I just bought a 1982 gs450l. New to bikes but I think I bought the best model Turning into more of a project than I thought but at least Im learning. So I have a petcock that leaks thru my air pods. I took it apart and stretched out the spring but it still leaks like crazy in res/on/ and prime. I am wondering if anyone has any good sites to find a replacement also if anyone has some cut-up-an-air-mattress trick or something to get me riding. Hope Im on the right thread.

You're in the right area but may want to start your own thread. Welcome to the forum and check your float bowls also.

[Zukimoto]

ok thank you

You're in the right area but may want to start your own thread. Welcome to the forum and check your float bowls also.

Ok i will start my own thread thanks.