I thought I had this problem licked a few weeks ago. Here in W.Va. the spring/summer/winter weather rotations make riding opportunities sporadic. Yesterday we had 70 + degrees and so was out for a couple hours on my relatively new (used) 2001 GS with about 4k miles.

Previously I had trouble starting and running but seemed to clear up when I went to "prime" on the fuel valve. I pulled the valve and found that it seems to need higher vacuum on the normal position than it does on reserve for fuel to flow. I also added a hose clamp to the outboard end of the vacuum hose from the carb. (I can't get to the carb end without pulling the tank, which I have been reluctant to do). So I have been leaving it on reserve and until yesterday, no problems.

But right after I filled up with gas, after riding an hour or so, it started running rough and I had to pull over. Got it started, switched to "prime, and it seemed ok. Then it happened again. So I had to cut the ride a bit short to head straight home, but it seemed to run fine again after a bit.

So far, all I can come up with is water in the gas, or something is interrupting the fuel flow, or maybe I have an intermittent vacuum leak. Maybe a cranky carb float valve?

Any ideas for further pinning this problem down?

Thanks a bunch, the helpful discussions from everyone on this site are great!

Check the little O-ring on the top cover of your carbs. The vac line has a tiny O-ring that is really easy to lose. I forgot this one time, and the Idle was all over the place, and sometimes it would just randomly die. hope it helps

well i don't know much about bikes ....yet! but a bit about cars, i would say check the filter in the airbox and make sure its all good and make sure you look all the airlines over for dryrot that would cause a leak. there is a filter in the gastank that may be filthy also i have heard alot in my gs500 research about bad petcocks (a.k.a. the fuel shutoff valve i believe) its strange that it happened right after the tank was filled maybe it kicked up some crap off the bottom of the tank. wait !! i was just looking in my "Cylmer" manual and theres a charcoal canister under the battery (page 255) that they say "Fuel vapor from the fuel tank is directed into a charcoal canister when the engine is stopped. when the engine is started this vapor is drawn,though hoses, to the carburetor and into the engine to be burned " if that cnister in bad for some reason or it has water in it, it may be giving you just that prob when you stopped it to fill with gas  :dunno:   they also say you should drain the "drain hose" for the airbox i guess it gets water build up, anyhow i hope that helped it was alot of typing

There's no charcoal can on this bike-that's CA only. The bike is like new, I don't think anyone has done anything to it other than oil change and maybe a valve check. The hoses are clean and smooth. I did drain the carbs, added some gas preservative when I bought it in Jan. as it was far too cold to ride. I guess there could be some crap clogging the filter in the tank but it everything seems clean. You would think after all these years, Suzuki could get the petcock and vac line working right. What about carb cleaner added to the gas? Maybe I have a sticky float valve?
Thanks for the ideas, I'll dig into it when it warms back up. Unbelievable that we're getting snow and 35 degrees today. It was 70+ yesterday!

What part of West Virginia? I live in Lewisburg and work in Covingtion, VA. Check the 'Meet and Greet' section of the discussion board. Several of us have planned a trip down the Skyline Drive on March 27. We'll be leaving out of Cumberland, MD on Saturday morning. As it stands now the trip is a go.

If your bike ran fine before the fill-up and started acting up afterwards I would strongly suspect water in the gas. If it runs poorly even in the prime position then it isn't the vacuum line. Gas flows through the Prime position weather there is vacuum or not, the prime position is just an open passage through the fuel valve.