Petcock not flowing enough

[Junkie]

I'm pretty sure it's been off reasonably recently, as the guy I bought it from has another tank missing the petcock and gas cap which I'm pretty sure was the original one - it was fairly rusty, and he put this one on when he got the bike about 2 years ago.

What is the seal made of?

[Dave8338]

Keep in mind...if there is even a tiny pinhole in the diaphragm, air flows easier than a fluid and the diaphragm is tiny. With a small volume of air, a small pinhole (s) and a drop in vacuum (as you have the thing nearly pinned), my bet is on the petcock being faulty. Even if it isn't, replace the roach...it is a safety part and in addition, will rule that out as being a possibility. :?

[Dogma]

What is the seal made of?

I dunno. The old one was some black formerly rubbery stuff. It was so hard it almost felt like ABS plastic. There's no chance somebody put the wrong thing in there, since it was molded to fit the grooves. The new one was also black, but the color doesn't say much. Lemme see here.... According to my Parker O-Ring Handbook (no, really) compound compatibility chart... several compounds are "satisfactory" for use with gasoline: Nitrile and Hydrogenated Nitrile, various flourocarbons (FKM, like Viton), and flourosilicone. Regular silicone is "unsatisfactory". If I had to guess, I'd say Nitrile. What happened to mine is classic for Nitrile. Viton might last longer, but another 27 years of Nitrile ought to be good enough.

I looked at the petcock you linked to earlier, and it is different from mine. Maybe it does use regular o-rings, maybe it doesn't.

[walt]

Does your bike have an inline fuel filter? If yes it could be dirty and need replacing.

[Nessism]

I still say measure the fuel flow if you have not ordered a new petcock already. Get a glass jar and put the fuel line in it. Turn the petcock to Prime and time how long it takes to fill. This is your baseline since you state the bike runs fine on Prime. Next, do the same test with the petcock on ON and RES, you will need to draw a vacuum on the vacuum line input to the petcock. If the petcock flows the same amount of fuel in both situations, it's fine.

A couple of things worth looking at are the screen filters in each carb at the fuel inlet, and make sure the fuel line does not loop upward as it leaves the petcock and flows into the carbs. Both will create restrictions in the line which will reduce fuel flow.

Good luck.

[robertbarr]

I had a similar problem with an '82 GS850. It would starve & die on level pavement at 60 m.p.h. I'd have to stop somewhere and let the bike sit on its sidestand for a few minutes, and I'd be good for another few miles. It really sucked. This would occur on either 'ON' or 'PRIME'.

I pulled it apart and didn't see any problem with the inlet screen, but since it was trashcan-bound anyway, I just gutted the screen and tried it like that.

That solved the problem.

[Junkie]

I still say measure the fuel flow if you have not ordered a new petcock already. Get a glass jar and put the fuel line in it. Turn the petcock to Prime and time how long it takes to fill. This is your baseline since you state the bike runs fine on Prime. Next, do the same test with the petcock on ON and RES, you will need to draw a vacuum on the vacuum line input to the petcock. If the petcock flows the same amount of fuel in both situations, it's fine.

A couple of things worth looking at are the screen filters in each carb at the fuel inlet, and make sure the fuel line does not loop upward as it leaves the petcock and flows into the carbs. Both will create restrictions in the line which will reduce fuel flow.

Good luck.

It does loop up a small amount, it would be very difficult to not have it do so at all. No filter though, and it should siphon fine - the amount of restriction present in the line is small enough that I would be very surprised about it causing any problems, and all that should really matter (once you get the line full) is the length of the line.