Should I just leave the  fuel supply valve pointing at "ON"  all the time even though I'm done riding?  I don't see an "Off" selection on the petcock, what is the "Fuel" selection for?? and when do I need to put  it to "Prime"?   Again thank's all.

Just leave it in the "on position" unless of course you are running out of gas, then switch it to "res". I dont use prime ever. Always fill the bike up with the petcock in "res" (at least this is what Iw as taught, it may not matter), but after remember to switch it back. When I was a new rider, I would forget that it was running on reserve tank, and it would run out of gas, I thought the bike was broken, then I would remember to switch it back to "on". DAmn i felt dumb.

You do NOT have to put the petcock on RES to fill it up.  ON is for regular use.  It picks fuel up from about 2" from the bottom of the tank.  RES picks fuel up from a little lower in the tank.  That's all.  Both of these are vacuum actuated.  In other words, the engine must be running to allow fuel to flow to the carbs.  PRI, or prime, allows the fuel to be gravity-fed with the engine off.  It is used in cases like when you run completely out of fuel and need to get gas to the empty carb float bowls.  Leaving the bike on PRI won't hurt anything unless you develop a leak or sticking float in the carb.  Then all of your fuel will leak out all over the place.  This will not happen if it is in the ON or RES position.

As a side note, PRI comes off the reserve fuel line, so when you run out of fuel you can position the lever horizontal (RES) or just all the way to the up position (PRI).
Take care, Eggert Johnson 

Also, if you have a fuel starvation problem at highway speeds, you might want to try riding with the petcock on "PRI".  I had this problem and this simple fix was all I needed to do.  Basically once you get to about 60-80 your bike feels like it's running out of gas and eventually stop.  I had to pull over everytime and wait a couple minutes then start her up again.  A big pain in the ass.

Dorianfes

Quote from: Dorianfes on July 12, 2006, 08:00:46 PM
Also, if you have a fuel starvation problem at highway speeds, you might want to try riding with the petcock on "PRI".  I had this problem and this simple fix was all I needed to do.  Basically once you get to about 60-80 your bike feels like it's running out of gas and eventually stop.  I had to pull over everytime and wait a couple minutes then start her up again.  A big pain in the ass.

Dorianfes

Yo, thanks man. I just got my 99GS little over a week and I first sign of fuel starvation and I would put it in RES and it would die on me(Highway speed). I will try that next time.

just be careful with that solution, the switch to PRI to stop the fuel starvation.  Since it runs on the reserve line if you run out of gas you are going to be walking or pushing your bike to a gas station and that sucks.  Anyone who has experienced that opps can tell you that.