I recently bought a '90 GS500E (my first bike) and after adding 200 miles to its 12,000 I've gotten a poor idle and performance from it after it's warmed (it slowly loses power when idled).  It seems to run better when the PRIME setting is engaged.  A mechanic told me it was likely the spark plugs but they have since been replaced.  I know near to nothing about carbs or jets so I wouldn't know where to begin if those are to blame.  Arg.


JB

Faulty petcock.  Notorious problem on older GS's.  Or possible vacuum leak someplace.  ON and RES are vacuum actuated.  PRI is not.  If it runs right on PRI, but no on the ON position, then it's one of the 2 things above.

200 miles, eh?  You're not still on your first tank of gas are you?  When the fuel level is WAY low the bike will run on PRI even when it won't run on RES or ON.  (Oh, and it can look like there's plenty of fuel even when there's not.  I know this from the first tank of gas on MY first GS500!)

Just a thought.

I can keep it running if I open the throttle though...but sometimes opening it up even all the way gets me only to 5,000 rpms or so...it just seems to run very inconsistently after warming.

Check your gas and fill it up. Put it in Prime while you ride it as glenn stated. Your bike may be stock as well, which means rejetting may be in order. Do a search on 'pilots' or 'brass' to find out if your carbs are stock. The search should describe where to look on the carbs to find out if they're stock. Then go from there.

I'm with Kerry here.

Second, check the vacuum hose that goes from the top of the left carb to the back of the petcock. Make sure it's well connected and there are no cracks or leaks.

See a picture here:
http://www.angelfire.com/mt2/mikesgs500/rejetting/DSCF0134.JPG