I have a 1990 GS and have been experiencing a problem when I'm riding at higher speeds. (It seems like I'm starving for fuel) I've taken off my tank and inspected the fuel lines, fuel screen inside, and vacuum line. When I'm in 4th or 5th and accelerating I will start to lose throttle and when I slow down it starts to pick up again. I thought it might be bad fuel so I put some 92 octane and still experienced the same problem. When I move my frame fuel valve from ON to PRI, the problem goes away. I just rejetted the carbs a few months ago (40 pilots and 125 mains). The plugs look good, and I've double checked all the lines for kinks and everything looks good. So if anyone had any suggestions please drop me a line. Thanks.
the petcock's on this bike is known to have issues..
you got to clean teh petcock screen.. or it wont work.
I also encounter something similar. The bike will run fine most of the time but somtimes it will run rough and seems to backfire. Fiddling with the petcock will normalise it. I have yet to clean the petcock or install a fuel filter yet. Is this problem common?
Yep, common problem. It seems there isn't enough vacuum at large throttle openings to keep the petcock open enough to provide sufficient fuel flow.
Replacing the fuel valve with a couple of simple on off taps is one solution.