1980 GS550 Petcock Operation Question

[Guest]

Hello all,

New to the forum. Recently bought a 1980 GS550; trying to get it ready for the spring. While trying to sort out starting issues I have directed my attention to the petcock since I found gas had enter the crankcase (already changed the oil and filter). It has the dreaded '80 no-lever petcock.

I have been searching the forum but so far all I see is "get a new unit". I am looking for some help understanding the operation of this "thing" to make sure it is bad before I buy a replacement.

What I know:
- It only has "On" (horizontal) and "PRIme" (vertical).
- Prime needs some cranks/vacuum pulses to start the fuel flow.
- The "On" position is vacuum operated, so fuel should not flow if the engine is off.
- There is no reserve.

When I turn the position to "On" (horizontal), fuel is still coming out. While testing it out of the gas tank I noticed that it will hold a vacuum if the hole in the front is covered (see sample pic below). That sounds to me like the diaphragm is still in good condition, but I am not sure if that helps in any way to understand why fuel still comes out while set to "On". If the opening is not covered air just goes in through there.

'80 petcock.jpg

Looking for any ideas on what may be happening with this unit.

Thanks,

[Nessism]

If fuel is flowing without vacuum then the O-ring on the diaphragm has failed. Another thing to be aware of is that the back cover has a check valve under a pressed in cover. If/when the check valve fails you won't have a functional Prime function anymore.

[Gumpster]

Good morning and welcome to the form,
This petcock mainly consist of the diaphragm, a spring, and an o-ring on the portion of the diaphragm that stops the fuel flow. You can probably remove the four screws and clean it up and check that o-ring for proper sealing and the spring for proper tension. The diaphragm might also be stiff restricting movement.

[Tom R]

Turning the slot to the prime position closes off an air bleed, located through the center of that slot. As long as the check valve holds, the vacuum from cranking is supposed to hold the diaphragm and fuel valve open. In the run position, the air bleed is open to the diaphragm and requires a constant vacuum signal (like a running engine) to hold it, and the fuel valve open.
Mine has never really worked right in prime, if the bike’s sat for too long, I have to pull the vacuum line off the carb and take a pull of it.... at least you know real quick if the diaphragm is in tact or not.

[e-square12]

Thanks for the replies. I will post some pics of the inside today when I get home form work. Maybe I'll use some Harbor Freight o-rings I have to see if that helps. It if does, I'll get the Viton ones.

[e-square12]

Good morning and welcome to the form,
This petcock mainly consist of the diaphragm, a spring, and an o-ring on the portion of the diaphragm that stops the fuel flow. You can probably remove the four screws and clean it up and check that o-ring for proper sealing and the spring for proper tension. The diaphragm might also be stiff restricting movement.

So.... I had found a spring while cleaning the workbench on Sunday and decided to save it. Turns out it was the spring needed for the petcock diaphragm to stay closed. Once installed, the o-ring pressed against the top and I was able to close that path. However, while testing the two positions I still noticed I needed to put my finger over the front opening to hold a vacuum. Playing around with a flashlight and a small nail, I found out the check valve was long gone. There were a few pieces of black goo in there, but no valve. Getting a new petcock; case closed. Thanks for the help.

Pic for reference.

20180206_212638.jpg

[mrbill5491]

That was the best fix you can do, is buy a new one.