Petcock issue

[azr]

I installed a new petcock on my 79' GS850. I had a bad idle before with backfiring, if I turned the petcock to prime the bike idled great. So I put in the new petcock and now she'll won't even start in the 'on' position but still runs pretty good in the prime position. Any suggestions? the air box has been rebuilt, the carbs have been rebuilt, valves done, timing done, new intake boots, the whole nine yards. Any help would be appreciated.

[BassCliff]

Hi Mr. azr,

It's a vacuum operated petcock. If it's new and doesn't work in the "ON" position, I would first suspect something wrong with the vacuum. Is there a leak somewhere?



Thank you for your indulgence,

BassCliff

[Turtle]

I know its a silly thing to say but have you got enough fuel in the tank to not need the reserve setting

[tas850g]

i am in the process of doing the same thing. Cleaning out my tank with Milkstone remover, which has worked great. My next step is to install the new petcock. If i can avoid any problems that would be great. I will keep following this thread.

[Dave8338]

Do you have the vacuum line hooked up? It is very unlikely that both petcocks would have the exact same problem. Have you applied vacuum to the vac line using your mouth? This is an easy and quick way to test the petcock.

[Zook]

I can hear Carl now....

"It ain't got no gas in it!"

[Spyder]

Start with the vacuum. Just went through the same battle with mine. Sounds like a vacuum problem.
Pull the vac. line off petcock and start the bike on prime, and check vac with gauge.
I'm sure Basscliff could tell you how much there should be.

After you get your vac. issue is resalved, you now have a spare(and likely good) petcock...

Good luck...

[argonsagas]

As others have said, the problem is almost certainly a vacuum-related one.

The standard Suzuki petcock has two small tube connectors, of different sizes. The larger is for fuel and the smaller for vacuum.

The petcock internal valve opens when vacuum is applied, and this allows the fuel to flow. Consider it a sort of safety device.

Adjusting the knob to PRI also allows the fuel to flow, as you already know, but you should not leave the petcock on that setting except when necessary.

On your carbs, usually near number 3, you will have a short tube connector point which is there to allow a hose to be attached. The vacuum hose will run from there directly to the petcock, where it connects to the smaller of the two connectors on bottom of the petcock.

Be sure you use the correct diameter of hose, AND be sure it has no breaks or even pin-holes. If it has a hole of any size or is even a tiny bit oversize then air will leak in and lessen the amount of vacuum at the petcock, which will then not function correctly.

[azr]

I purchased the petcock only to find out that it installs backwards (should have bought OEM). So the fuel and vacuum lines are longer than original. The extend towards the battery and then lop back to the petcock. Much longer. I thought that might effect the feed but it runs fine on prime. I thought maybe there was not enough fuel in the tank so I put it on reserve and had the same problems. I'll check the vacuum on my first day off.