Huh, that's an eye opener, thanks.
I'll check them tomorrow first thing in the morning.

Alright, here's the update:

Opened up the carbs once again, cleaned the jets (they WERE a bit clogged, especially on one carb).Closed her up, she fires up and chokes up to a healthy 3K RPM, no breeze from the engine or carbs, no pulses, smooth as can be...BUT! When I let the choke down she stays at 3K RPM, as if the choke is still on, and when I shut her off and start her again without any choke she fires up to about 1-1.2K RPM, but if I use the throttle and get her to higher RPMs she'll stay at 2.5-3K again.

Is this a throttle/choke cable problem? I tried firing her up and while in 1.2K RPMs wiggled the hadlebars to the left/rigjt and it made no difference until I added the throttle

try backing out the idle screw to lower the rpm.

This could still be a vacuum leak somewhere around the intakes.

I might also suggest lubing both the choke and throttle cables and checking for appropriate slack on both. If there isn't quite enough slack on the throttle cable, for example, it might be hanging open a little.

Alright, made it worse again.
Tried the "hillbilly carb sync" method (gas tank off yet attached, bike running, taking off the plug cover and seeing if there's a difference in power). Long story short - I unscrewed the whole idle screw off by mistake, and now have to tear her apart again and install it...she didn't run at all after I touched the carb screw, not with thottle, rarely with choke....*sigh* if only I could afford it I'd tow her to the nearest workshop and have them fix her for me...

Before you pull your carbs off again, do you have a Haynes or Clymer manual that covers your bike's year? I would STRONGLY advise reading and trying to work within the methods presented there to avoid making things worse.

It seems to me like you're trying things with a wish and a prayer that things get fixed and without any idea whether or not it could/should/would help.

No manual but the Owner's manual that came with the bike.
People told me it might be a vaccum leak from the air fillter. I sprayed it with carb clean and didn't see any difference in the RPMs but it does have a tiny crack on one side.

If you buy nothing else for your bike, buy one of these http://www.amazon.com/gp/product/1844258815/ref=as_li_tf_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1844258815&linkCode=as2&tag=gstwincom if you're going to do any work yourself.

If I didn't have mine, I wouldn't even attempt to do any work. Good luck trying to solve your bike's issues.

Alright, I downloaded the manual and read the carb part. Overall - I wasn't that far off. After the bike started, I should have checked the throttle cable slack and be done with it.

I'm going to put back the sync screw, set the throttle cable slack and sync the carbs. It should solve my problem.

**UPDATE**

Throttle cable is spot-on, the diaphragm are equally placed and return in the same speed. Choke cabew was a bit loose and didn't return all the way slowly so I adjusted it. Noticed a slight leak in one of the manifolds between the engine and carb, that needs to be replaced and maybe will solve my problem

Choke stuck open is a     

It is possible to reach the metal bit the pulls on the choke needles.  Reach up there and and manually push the choke needles in, see if that helps.

Alright, the choke is alright, I fixed it and now it comes back to place like it should. The airbox and engine gaskets show no signs of leaks, the cables are alright, the throttle is responsive, everything's in it's place...so WHY won't she start? I hear the starter rotating but no ignition, choke or without it. The gas petcock is open (of course), and the battery is showing signs of weakness (probably due to me cranking the starter so much)...what now?
I'm seriously thinking of just having her towed to my mechanic and paying him next week when I'll get the paycheck...

Yet another stupid newbie question

Have you checked the compression? If you have ruled out everything BUT the cylinders, then that would be a place to look.

Actually, the battery is showing signs of weakness, and the dashboard lights start fading when I crank the starter, meaning - it's not charged! Now I'll have to wait until Sunday (we work on Sundays) to get it charged in a local shop, or just tow the whole bike there

I don't have any dash lights anymore, maybe another user can comment whether their lights dim. I think it might be normal.

You should be able to take your battery to an auto parts store and have them test it for you for free.  For sure get it tested before you pay money to replace it.

Quote from: lucas on July 01, 2016, 05:54:11 PM
I don't have any dash lights anymore, maybe another user can comment whether their lights dim. I think it might be normal.

You should be able to take your battery to an auto parts store and have them test it for you for free.  For sure get it tested before you pay money to replace it.

No,it's pretty much a brand-new battery, no need to have her replaced. There's a bike garage in town that can charge her for free

Alright, it's running again. Clutch in, push the starter and it idles nicely at 1400 RMPs when warmed up. The idle screw doesn't work for some reason.
The throttle response is a bit so-so, and when I rev it up to 3-4 K it comes down gradually within a few seconds, not immidiately like it used to. The throttle cable snaps into place as it should. Rode her in the building's parking lot, the throttle response is poor and the acceleration is delayed.

What could be the problem now?

Its a little hard to know where to start but, what were the jet sizes?  GS500F idle jets are notoriously lean at size 17.5, replace them with 20's so the choke is only required when it is actually cold. Lubricate throttle and choke cables.

Carb cleaning: I use engine De-greaser/wash, it cleans well overnight and doesn't damage rubber and plastic bits.  The good old carb cleaning dip is fine if its metal only that you are working with, and you don't splash in yur eye.

Is this bike new to you?  I think a flush of the fuel tank and petcock might be in order and a compression test and valve adjustment so that you are starting from a known place.  A 15 tooth front sprocket if you wanna wheelie