Do yourself a favor and lose the vacuum petcock setup. Lose the hoses. Lose the canister. And cap off the open vacuum nipples. Then run the petcock on "prime" and just remember to turn it to "on" or "reserve" when parked. That'll eliminate about 10 leak sources. Plus you'll have the ability to control flow in case of a flood, and limp home if need be. Personal preference. I also have both a bandit 1200 and gs500. The 500 has tps, but likely that's not your issue. If I remember correctly, the 500 has no tiny o rings on the diaphragm cover. The ports are built into body and are a pain to sync because of it. There's so much it could be... make sure the mixture screws are set 2-3 turns if drilled out already. I've also seen the enrichment plunger deteriorate on mine. Worth a check. Try running with "choke" on. If it stays running you'll know it's not electrical. Check o ring on carb manifolds too. Also if you find yourself disassembling the bowls again, do research on bigger jets. I get 70mpg with a k&n, exhaust, and bigger jets. Runs so much better. Also research a quicker ratio throttle tube. Good luck, i love my 500, my commuter bike of 5yrs all for $1k.
