I got a 91 GS500 from a shop. It runs great but i noticed something strange at low rpms. If i hold it between 2k-3k rpms it wants to die as if not getting air or fuel. I figured it might need a carb clean so i added a lunch box, R6 Muffler, and re-jeted 40/150. I also cleaned and set the fuel mixture at 2 1/2 turns and it runs great, idles fine, pulls and revs great but i still have the same issue. What could it be?

Could be the frame petcock. Switch it to Prime and see if it still does it. I have a brand new peacock in a bag off an 09 if you find yourself needing it

09 petcock won't fit an 89-00 frame.
Prime is good to try in case the vacuum is farkled or the petcock diaphragm is foo bared. If so, you'll need a new 89-00 one.

I have noticed something very similar. For me, right at 2k rpm, the engine almost knocks as if it shut off at that spot.  It has trouble going past 2k if you give it very light throttle across that rpm. But if you go up or down on rpm, it's just fine. It seems at its worst when the engine is cold.

I'm thinking this is probably worthy of a quick video to capture what I'm talking about. But I won't be able to make one till early next week with my work schedule right now.

- Bboy

Aww hell, a little fabrication and it will work

I have a problem with the frame petcock. the bike only runs on prime but the same issue. Well it work if I go strait from the tank to the carb and cap off the other line?

I just rejeted and I'm catching hell on from my low rpm launches.

I'm running
40/150
1 washer
2 1/2turns

I followed the instructions but after riding for a minute it almost wants starts acting as if it's running too rich.

any ideas?

If you only run in prime, then you're missing the vacuum line to the petcock.
Either fix it or just leave it. No need to start capping off lines. That won't fix your problem.

ok. got it Fixed.
It was the needle in the carb. When i rejeted i added one washer. I took the washer out and moved the clip to the clip from the third line from the top to the second line and it solved all my problems.

T issue with my petcock was the lines where crushed next to the body when the airbox was changed, now it works too! I feel silly how easy it was to fix.

I get great power ans stability across the whole band now. Thanks guys.

Haha funny how those little things will get you. Surprised you didn't notice a fuel overflow come from somewhere with the lines pinched up like that and you running it on prime. Glad you got it fixed!