I recently bought a 1991 gs500, which ran fine until I brought it to get inspected and it needed new fork seals. I picked it up last night (it was there for a week while they ordered parts) and it was running awful. When I let off the gas and then tried giving it gas again it would just sputter. At one point it did stall and took some convincing to get it restarted. I tried turning the petcock set to reserve, with no better luck.
By the time I got it home and parked it, it was idling at around 4k.
When I called the shop this morning they said that they didn't do anything to the engine and that it was running poorly when they got it (although it ran fine when I rode it there).
Any guess of where to start? Perhaps without disassembling much.

Thanks in advance

probably not it, but first thing i'd check was my gas. the sputtering symptoms sound similar to the first time i ran out of gas..

no, plenty of gas. i wish that was the problem 

possibly a clogged petcock? 

Mine does that on reserve. Need to clean it out.

A '91 is only 2 years younger than an '89.

I would suspect that the little (tiny) o-rings under the diaphragm caps got old, dried up, cracked and generally deteriorated.

Or, the carb boot o-rings (carb-to-engine), or just the carb boots themselves, cracked.

i rode it this afternoon and it ran pretty well. i didn't notice any sputtering or bogged down feeling, however it is still idling high.
it idles fine when cold, but once it is warmed up it idles at about 3,500 rpm.
anyone live in albany, ny and feel like taking apart a bike?

I would suspect idle set too high.

On the hottest day of the year, after a 15 minute or more ride, set the idle to 1300.

As the weather cools later in the year, you will need to use the throttle to keep rpm's up.

Or, you could just keep resetting the idle every 4000 miles (the suggested mileage to do oil changes, valve adjustements, carb synchronizations), or as the weather changes.  Have fun!