Throttle doesn't return to the stop with engine running

[Pontiac Collector]

I have a recently rebuilt 1968 Firebird carb from Cliff and after a little driving I notice the throttle doesn't return to full stop so at stop lights it wants to idle 1500-1800 rpm.  I can rap the accelerator and it drops to 800-900ish.  I checked it tonight and it returns to full stop with no problem when the engine is off, but when its running it seems to be held open either by binding or engine vacuum which sounds silly.  I am sure a stiffer spring would work, but that seems more like a band-aid and I'd like to get to the root cause. I am sure this is a simple one and thanks for giving us a forum to discuss such things.

[makomark]

Check to ensure mounting nuts are properly torqued - if loose, vacuum leaks will do similar things to idle speed. If mounting nuts are secure, see if idle returns to normal when the throttle cable is disconnected; if it does, then find the worn spot in the accelerator linkage

[Pontiac Collector]

Thanks for the reply.  The thing is I can physically see the throttle not returning to the stop when the engine is running.  When the engine is off, everything works perfectly.  It does seem like vacuum in a way, I double check that as well as proper mounting.

[makomark]

If all is secure at the mounting nuts, then check the fast idle mechanisms and adjustment. Something could be binding there, a clip could have came off, etc. Also, make sure the choke opens with the engine warm - if it doesn't, the fast idle cam stays engaged and you have the fast idle speed.

[Cliff Ruggles]

I would also check to make sure there is some clearance between the secondary throttle shaft arm and the link.  If the link is touching the upper section of the arm, it can hold it open slightly when the throttle is returned to idle......Cliff