Vacuum petcock

[letchworth rider]

1981 GS750EX I have the original vacuum petcock. It works fine as far as riding and running, does not leak through when sitting.
Prime does not work. I did put a kit in the petcock a few years ago. What makes the prime work? Just makes it a long crank after sitting.
Thanks.

[Eli69]

I have a couple around here somewhere. If I get a chance I'll take one apart and see if I can see what would cause that.

[Eli69]

One thing I can say is that one of my bikes seems to have some gunk that settles around the intake and keeps it from getting enough gas to run. Being lazy I just pull the hose off and blow through it to clear it out. Some day I will probably take it apart and clean it up.

[1978GS750E]

I believe in the "prime" position on your petcock, basically the lever is physically pushing against the spring that holds the diaphram closed. Causing it to allow fuel to flow.

You have to be very careful when rebuilding, to make sure you have the correct kit, as there are slight differences in some of them. (In my experience anyway)

But, I've been wrong before!

[salty_monk]

Does Reserve still work? I think you can put the bit inside in backwards & it will block prime & basically make it run from reserve the whole time... Dragging up an old memory here so don't shoot me down if I'm wrong!

[letchworth rider]

Here is where I am at. Getting repairs done when snow is still on the ground. Ordered a fuel hose and spring. When I get them I will drain tank and check it out again to see if it passes fuel before tearing it apart.

[letchworth rider]

So I pulled the petcock today. It is the model with NO lever. In researching a new one they do not come with a lever. Tried different positions with it off the bike using pressure and vacuum on the outlet. No fuel flow unless vacuum is applied to open the valve. Seems to be specific to 1980 and 1981 model years.
Been researching replacements. All outlets for fuel are in the wrong position on models with a lever. Guess I put it back together and ride. Works OK except for prime. No reserve on this either. Guess that is why there is a good fuel gauge on the dash.

[rphillips]

Those petcocks, for me, were always very aggravating to find PRI. It was never obvious which way to point the slot to get it to PRI position. I don't know where you applied pressure to the petcock, when off the bike, but when off the bike, and no pressure anywhere put a fuel line on outlet (larger) fitting, and while sucking on the line rotate the slot till vacuum is lost and that should be PRI. location... Also, the back plate of most GS petcocks are square, I'd think the outlet fittings should be able to be rotated to four different positions

[Nessism]

When in doubt, check the service manual. It describes how the leverless petcock works.

For prime, you have to set the slot to the proper position, then pull a momentary vacuum (hit the starter) to pull the diaphragm open. Then, a check valve in the cover keeps the vacuum from bleeding down, and closing off the valve. Gas will flow until you turn the lever/slot back to ON. Problem with those petcocks is the check valve fails. Particularly the crappy aftermarket versions of that petcock. I don't think there is any way to rebuild one, unless they gave you a new back cover, with check valve.

[Gorminrider]

Yes, if I'm looking at the right manual for a (16valve?) 750, they don't mention reserve or prime and the petcock is quite different to look at,..flatter and closer to the tank, while the 750T has a "normal" petcock with RES ad Prime...

If I had to guess, a "normal" might fit per the holes on the tank(but double-check-we can measure) but it just might not fit in the space available.The normal hangs further down than yours and takes up more room in that dimension, even if you got some longer hose where its outlets don't match your original...

I do have a bike with no prime so after it's been sitting awhile with carbs drained, I have a piece of hose and and a pet-hypodermic to apply some suction to the vacuum port...if it's easy to get at yours to pull it off, that is a little less ...self-effacing?

[Gorminrider]

the back plate of most GS petcocks are square, I'd think the outlet fittings should be able to be rotated to four different positions

I have wished hard it was so, but No.... You can remove the backplate and thereby rotate the vacuum port, but not the inlet or outlet.

[Nessism]

Hum, found this in the 850 manual. I thought it was in all that used this type of petcock...

leverless petcock by nessism, on Flickr

[Gorminrider]

I have this from the manual I have .... similar ...can't tell if it's the same but I don't see a "prime screw" or any mention of RES

Screenshot_2023-03-13_09-17-19.jpg

[letchworth rider]

So if I read this right -turn to prime, crank the engine, and a check valve will hold vacuum on the diaphragm. Even without the engine vacuum supplied, So you must turn the engine over for fuel to flow to prime the carb.
When on only -the fuel will turn on with vacuum, but shut off without vacuum.
Sound about right?

[rphillips]

Yep, that's the way I'm reading it in post # 12. Guess'in that's why I always had trouble finding the PRI position, I didn't know it had to have vacuum to get it turned on (started).