Yes, that manifold gasket. And if it's so crispy that you are afraid to take it off of there to try another position, it could very well be leaking a tiny bit of air and holding your idle up. Should not be like parchment!

Mark - The petcock is brand new and for sure shuts off.  I have the tank off of the bike right now with fuel in it and there are no leaks coming from the petcock. 

The gasket is like pressed on.  I cant even get a blade behind it to try and take it off. Any attempt would destroy the gasket. I think I will order one from the guy on Ebay and replace it just to ensure there are no leaks.  I ordered a bunch of carb parts to replace those currently on the carb - hoping that will stop the overflow issue.

Gents -  I have replaced the manifold gasket and the throttle cable as it seemed to have a small kink in it at the throttle connection, cleaned the carb and replaced all of the jets.  Started the bike - fired second kick and I still have a high idle.  I moved the throttle cable around at the handlebars and it seemed to change the idle higher, but not lower.  I don't see where I have a hang up with the cables under the tank.  I can't figure it out.  It's just too dangerous to ride with the throttle hanging up in a high idle position.  Oh and if I adjust the carb idle or air screw, they do nothing unless I screw the air screw all the way in and it slows the idle a little... go too far and it comes back up again.  Stumped in Sacramento. Any help would be greatly appreciated. Are there any mechanics familiar with the RT3 in the Sacramento area that might be willing to take a look at it and help me out?

With your new throttle cable installed , do you have freeplay on the cable , at the carb .
Some cables don't allow the slide to bottom out .
When the slide isn't fully bottomed out , the air screw adjustment isn't very effective .
If it's a cable issue , you might have to shorten the outer sheath slightly , on one or more
of the cables . You might have to disassemble the cable assembly , to determine which
one is causing the problem .

Without reading 5 pages again, has the needle jet been out & misplaced so its sitting on the guide pin rather than fully seated, & then it'll sit up in the venturi & the slide will bottom out on that instead of right down. Doubt the Oring holder-jet setter would tread in. Was just random thought.

Throttle should close with a solid "clunk".

True Mark, but would still clunk if it landed on a high needle jet. 'i think' remove carb & look through from the engine side & see if the slide is bottomed out & no light seen under it with the speed screw wound out so not contacting the slide. I dunno, i'm just plucking random thoughts on bloody miserable wet dark cold day. Have a happy dayyyy.

We posted at the same time Morley...  I wasn't responding to your post. 

I agree that it could be something like that if jet setter washer was missing for example...  seen that more than once.

Whats the gasket on the threaded cap that holds the throttle barrel look like? perhaps a couple of them stacked up... Thus, raising the the throttle tube and no way to allow it to close more... Just a thought..
Sneezles61