Okay so I have owned my 2001 GS500 for about a year, bone stock except for a Uni foam filter and one washer under the carb needles. Over the past few months the idle seems to have gotten gradually worse, where it would take a while to warm up to a proper idle. And by a long time I mean 15 minutes of riding. If I turn off the choke, the idle never settles when I pull in the clutch at a stoplight, and the bike stalls. Then when the bike does finally warm up and keeps from stalling, I notice that it is idling too high. You know the type, when the revs drop slowly with th clutch in, and sometimes it jumps up to around 3000 rpm.   

So I adjusted the mixture screws to 4 turns out, thinking this would solve it. It helped a little, but it didn't work. Then recently I took the carbs out and cleaned them, thinking my idle circuit was dirty. That didn't work either. Then today I got a vacuum gauge and I figured that I could just synch the carbs, but my vacuum readings are craaaazy. I should explain that I could only test the right side carb, using the OEM vacuum hose, because I do not have a hose that fits over the left side carb port (you know, the one that is normally plugged with the little rubber nubbin).

The vacuum gauge has a stop point at about 3 o'clock, which keeps the needle from measuring excessing pressure or excessive vacuum. While ilding around 1600 RPM, the needle is oscillating rapidly counterclockwise and then clockwise. It is doing this so fast that the needle usually looks to be around the stop point at 3 o'clock. It seems that the engine is pulsing pressure and vacuum waves back and forth, like maybe an intake valve is no closing all the way? If I rev up the engine, the vacuum smoothes out a bit around 10 in/hg, but still vibrating a little.

And the worst part is.... I haven't ever adjusted the valves.  I have owned the bike for 4500 miles worth. I have no knowledge of whether the previous owner had them adjusted. So I would like to assume it is the valves. Does anyone with vacuum gauge experience have any suggestions? Could this be anything besides the valves. I am almost positive there are no vacuum leaks, as I just recently had the carbs off, and checked all the connections. 

How many miles does the bike have total? Valves way out of adjustment would cause crazy readings when trying to sync the carbs, and the screwed up idle. Why not just check the valves----I think that would be far easier than what you have gone through so far.

you have a vac leak....spray carb cleaner around the carbs and see it the bike revs or dies

Bike has 20,500 miles total

do a compression test to see if your compression is low
if in spec....valves good enough
if low.....
pour a dab of oil in the plug hole and retest compression
if low....valves
if goes back up.....rings

Don't have a compression tester.

at 3 o'clock means nothing??

what is the vac reading?... in inches of Vac

why cant you hook it up to the other carb?
to adjust you need hooked up to both carbs at same time

The needle wasn't settling on any particular value, it was oscillating between the extremes of pressure and vacuum. This gave the appearance of it hovering around 3 o'clock, where the stop pojnt of the gauge is located. Kinda like the stop point that a speedometer needle in a car rests on. I am guessing the maximum vacuum reading would be around 35-40 in/hg, but that's irrelevant since the needle is moving too fast to be measured.

I didn't bother adjusting the carbs because the readings I was getting from the right side were so screwed up. There is no way to adjust the synch screw if the needle isn't holding still.

I did not get to check the left side carb because I do not have a hose small enough to fit over the port. The only reason the right side port worked okay is because it has the OEM vacuum line firmly in place already (I hooked the other end up to my vacuum gauge). While testing the right side carb, I had already re-plugged the left one.

Dropped the bike of at a local shop, that way I can relax. They will do a leakdown test and check the vacuum connections, and let me know if they think the valves need to be adjusted. I was going to just do the valves myself if needed, but they quoted me only $75 if they do them. That just may be worth it....

I had this exact same problem. The remedy for me was to have the valves replaced. The edges of the valves had delaminated and it gradually got worse and worse.

Got your Pm nick_the_man ...
But ... I almost think its very very very hard to burn a valve on a 2 cyl with 1 valve per chamber. You see, as the valve starts to hang open, a precursor to burning unless the valve was made of ... sheite, which they are ... but it has to be like diahorrea sheite and not regular sheite ... the bike will quit running on that cyl. With a pair of cyls and you're barely pushing 500cc, you aint going no where long enough to burn it. It can still burn, but that could be due to the fact that you got shietty valve ... and yes you got diahorrea sheite while the other GS'es have been blessed with prefect sheite.

Yea sixspeedsquirrel - Give it to a shop, that would be the perfect way to ensure you never can relax ... really to relax, you need to be under the F*(ker looking for stuff taking pics and posting here.

Cool.
Buddha.