I'm posting this under your initial post because things have gotten pretty far ranging and more conditions and qualifiers have emerged---there have been lots of good postings but to solve your problem you need to get back to the basics--your high idle is a dead give away to a vacuum leak---provided certain other things are ok. The post regarding the throttle stop on the manifold is a first to address. The second is to be sure the timing is not advanced too much at idle speed (hard to do when the motor won't stop racing) -- set the timing with a test light at a moderate 0-5 degrees BTDC with the motor off, so there's nothing else effecting it. Plug the vacuum for the distributor and power booster if you have one. Run the engine to see if you've slowed it and check the timing with a timing light to see where it is dynamically (at idle it should still be where you set it statically). If you have slowed it, plug the vacuum units back on one at a time and see what happens--then use any of the "spray" methods to check for vacuum leaks--carb cleaner, propane, water--all will work to effect idle when the speed is actually at an idle speed. Don't forget to spray the intake manifold itself--I've seen both aluminum and cast iron manifolds with cracks. Question--not that I'd expect it of a 2 bolt carb, but do either of your carbs have a small spring loaded valve built into the throttle plate?--often a source of trouble. I'm happy to see that setting your jet to an eigth inch depth helped--that's only a general sort of spec though--the method of 12 flats down to start off is equally valid.