Update: Problem solved. Unfortunately, since I adjusted several things at once, i.e. added an additional carburetor spring, took up some slack in the throttle & choke cable, etc., I can't say for sure where the problem was. Mechanics 101, right? Thanks for the help!

Don't put it IN your ear... just real close ... I don't think ya want to suck something outta yer head!

Not knowing what carb(s) you have it could be wear on the throttle shaft(s) if you have a HS4 carb or HS2(s), if you have a HIF carb possibly worn seals. With wear the carbs suck air through the throttle shaft to check this you need about 3 feet of tubing (1/4" fuel hose or similar is fine) and place it around the ends of the shaft while the engine is running and the other end in your ear and you should hear a sucking noise, if you move the shaft around and the noise changes it confirms the leak there. You could also spray around it with some ether and see if the revs rise which will also indicate an air leak.

I'd suspect an air leak. If the idles screw(s?) are backed fully of it shouldn't idle at all. I'd also suspect the dashpots are not falling smoothly, possibly due to a gummy main jet, or the needle not centred. With the engine running, unscrew each dashpot cap one at a time, and slowly pull it out part-way, but holding onto it. Then while holding the cap steady up about 1/2" (the damping action should still work if you hold it still), rev the engine and then let it return to idle. If the rpms are dropping slowly, push the cap in, observing if the rpms drop more quickly as you push. Your push is providing some assist to the dashpot spring.

I forgot about the lift-pin (my carb doesn't have one). If you do have lift-pins, with the engine off, unscrew the dashpot caps and test with the lift-pins to see that the dashpot cores drop properly.

any other work done to the car recently? feels like an air leak to me.. 

When it does this, try partially choking the intake of the carb with your hand see if revs drop..

The last time I had driven Rosebud (4 months ago) she was running great (ref. post “Drain Float Chambers?&rdquo. Today I primed the oil pump, put a squirt of motor oil down each spark plug hole and turned the engine over until the oil pressure came up. I put the plugs back in and she started nicely and settled down to a smooth idle. As the temp rose I was revving the engine a bit and noticed it took a long time for the engine to settle down to an idle—like 2-3 seconds!

I’ve checked the following things:  1) no kinks or resistance in the throttle cable  2) throttle springs tight and no binding in the throttle linkage—also happens when I move the throttle linkage at the carburetor with my fingers  3) choke is completely off  4) idle adjustment screw in full off position.  5) dash pots full  6) no apparent vacuum leaks.

As I said, she revs nicely once warmed up and idles well, it just takes “forever” for it to return to an idle. Any ideas?