Theres on and pri and res but where is off??  I am finding on to be the off and pri (primary) to be on.  The bike doesnt want to start in the on position.  And pops just is sure that the on really means no! lol thanks guys.  Im new to the Gs and riding bikes. 

There is no "OFF" setting. This is controlled automatically by the insides of the fuel petcock.

This schematic might help to illustrate what's going on:


Inside the petcock is a diaphragm that acts as an on-off valve. When the bike is not running, the diaphragm blocks the fuel passage from the "ON" inlet. The "PRIME" setting bypasses the diaphragm and allows fuel to flow even if the bike is off.

If the bike is running, a vacuum line applies vacuum pressure to the diaphragm and pulls it away from the "ON" inlet. This allows fuel to flow from both the "ON" and "PRIME" inlet fuel hoses.

The schematic was drawn to clearly explain the concept; it is not shown exactly as in reality. See the following sites for actual petcock photos that might help to further clarify the issue:
http://gstwins.com/gsboard/index.php?topic=11722.msg98161#msg98161 and
http://www.bbburma.net/FuelHoseRouting.htm

Hope this helps! Good Luck!

edit - resized picture

Thanks that helps.  So when the bike is not running should I turn the petcock to the on position?  And when its running in the prime?

I say keep it in the 'on' position as much as you can; that's the way it's meant to run.

Many older models (before '02?) have fuel starvation problems when set to 'on', because the fuel petcock is of poor design. In this case, you may want to keep it on prime. Otherwise, only use prime if you drained the carbs for whatever reason and need to feed them some fuel so you can start the bike.

In your case, if the bike just doesn't want to start when set to 'on', keep it on 'prime'. Personally, I'm not aware of any negative consequences to using 'prime' all the time. You will, however, be lacking a reserve, since 'prime' drains from both the main and the reserve, so keep a close eye on the mileage on your trip meter.

+1 on what the other guys said.
Anyway, Domindart, we share the same model year GS ('01), I had no issues about the fuel system so far (bought my bike used last March and rode 8000 kms = 5000 mi on it), so I'm quite confident you'll have no fuel starvation problems too (I hope).
Anyway, during everyday use, the GS petcock is only meant to be set either on ON or RES.
PRIme, as already pointed out, is only to be used when you need to fill in the carbs' float bowls again after winter/longtime storage, or if for any reason you have no more fuel into the bowls. Or, besides, when encountering fuel starvation problems (hopefully, not our case ).
What's important, never forget the valve on PRI with engine off. The vacuum system thus being by-passed, fuel might flow freely into your float bowls, overfill them and get into the airbox (not so good) and engine/crankcase (worst case, definitely not good)

There's another valve performing ON/OFF functionality, the small tank petcock located under the fuel tank (left side), only accessible when taking the tank off, and operated w/ a flat-bladed screwdriver.

Maybe your bike won't start now on ON because fuel level is low? Try switching to REServe. Just my guess.
Hope it helps.

Ill check fuel level.  But I remember it not wanting to crank up while at on.  Switching to Pri it started right up..

If it only gets fuel in the PRI setting,  check your petcock's vacuum line,  sounds like it needs to be replaced because of leaking which is preventing your system from working properly.

Yeah, or maybe the tank just needs more fuel. 

Here's some data gathering you can do:

    1) See if the bike will start/run in the RES position.  (Let's assume that it won't.)
    2) Add half a gallon of fuel to the tank and try the RES position again.
    3) If the bike starts/runs fine, you were just low on fuel.  If it doesn't,
        check that vacuum line.
    4) Now switch to the RUN position.  With only half a gallon added, it should die
        once the fuel lines and float bowls empty out.
    5) Add another gallon (or go ahead and fill up).  If your vacuum line and fuel
        switch diaphragm are OK, the bike should now start/run fine in the ON position.

Ran the bike today and started in the res position as you, Kerry, suggested I try.  It started fine in the res position.  The bike was nearly full of gas.  After a ride in the res position I switched it to on and it seemed stutter a bit but it cleared up on the ride home and ran great.  So far no probs I guess, I will update if I have any more problems.  Thanks for the help.