I have a 2004 California version GS500F.  It was running fine till it sat unattended for nine months. 

Symptoms:
When cold, chock on, bike starts normally.  Idles about 5,000 rpm.  Then choke off, idle comes down slowly, then dies.  If I try to start the bike right away, it won't start.  I have to wait about a minute.  Then it starts and idles proper, maybe a bit bouncy on the tach.  I can rev it, but it's very hesitant to come back to idle.  Then it dies.  Then I have to wait a minute to try and start it, or else it won't start. 
I took it out for a ride around the block.  Power is good, for a minute, then it starts to run on one cylinder.  Then it dies.  I have to wait a minute to start it.

Treatment:
I've taken the carbs apart three times now.  I have not adjusted float settings or synch'd them.  They should still be factory set. I have taken the pet cocks out of the equation by making a temporary fuel tank go straight to the carbs.  I have taken the Pulse Air system out of the equation by capping all the lines that go to it.  I have taken the vapor recovery system out of the equation, except where there is a tee in between the carbs that goes to the canister.  I have taken the vacuum control valve out of the equation by capping the lines going to it (and I've tried capping and uncapping the two white things it goes to on the forward top part of each carb).  On the bottom of each carb, there's three exposed brass jets that I've cleaned.  The float bowls and float valves look new.  The diaphragms look new.  It has fresh gas, new plugs, decent air filter.  What am I missing?  http://gstwins.com/gsboard/index.php?topic=46124.0 helped me figure out all the spaghetti under the seat.  There's a vacuum hose diagram about halfway down this page: http://gstwins.com/gsboard/index.php?topic=56491.0

Thanks for reading.

If you rev it ,and it does not come back to idle right away you can have a binding cable,but more than likely you have an AIR LEAK.
An Air leak is the #1 reason the idle don't drop back like it's supposed to.
Spray some carb cleaner around all of the boots while the bike is running..If you hear an increase in RPMs you got a leak....pinpoint it ,and fix it.
-CS

Cables are not binding. 

I sprayed half a can of starting fluid on it: no change...

A hanging idle is a symptom of too much air ,and not enough fuel....so if you are certain you don't have any air leaks, your carbs or petcock or fuel lines may be a culprit. Dirty tank,or collapsing fuel lines pinching off the supply ,or the petcock could have become gummed up during the sit.
Try a new* fuel line from a different fuel source (such as an extra tank)..this will bypass the petcock ......gas tank,and fuel lines all at the same time.......and try it maybe you can hear  a change.
BUMP,maybe some of the experts can help more than I.
-CS

Replace your plugs. they are 2 bucks each, just do it.



OR you didn't correctly clean out your idle jets. 9mo. is about 7 months of sitting more than it takes to completely varnish and grit up your idle jets.  Take the carbs off again, remove the jets, soak the idle jets in some cleaning solution, like a cup of carby spray.

Then take a braided copper wire, just small enough to fit in the big end of the idle jet, jam it in there, hard, and twist.

you may see a shite ton of cripe come out. then take one strand, small enough to fit in the jet hole, and stick it in and wiggle. then blow the whole dealio out with a compressor. take care not to turn the idle jet into a rocket, and lose it forever.

If your idle mixture screws have the little brass plugs in them, remove and reset the plugs to 3 turns out.

I bought my bike brand new, and had this issue right off the bat.  Took it in to have them look at it since it was under warranty, and they fixed it.  When I asked what they did, their reply was "a carb adjustment".  I was in a shirt and tie at the time since I just came off of my office job in a skyscraper downtown.  They must have pegged me for a weekend rider or a noob.

When I rejetted a short while later, no brass plugs.  So they reset the pilot air screws.  I'd do the same.

Quote from: jestercinti on September 11, 2012, 06:14:11 PM
If your idle mixture screws have the little brass plugs in them, remove and reset the plugs to 3 turns out.

I tried that last night.  Thank you.  One was 2.75 screws out and one was 1.25.  And the brass plugs were still in.  Alas, the problem remained.  I'm down to swapping carbs from my dad's GS, and go from there.  I thought this was going to be easy...

Check your valve clearance.  Does it backfire?