Brian...
What were the symptoms of the idle problem that led you to swap carbs?
How long have you owned the car?
How many miles on the engine?
When did the problem start occurring?
Did you do any work to the car just before the problem began?
Did it begin all of a sudden or is it something that has gotten worse over time?
Was the clutch take up point that deep in the stroke before you changed the clutch and had the flywheel lightened?
Is the idle drop consistent every time you dip the pedal, or are there times when the idle does not fall as much?
Is the throttle linkage free and unbound?
Does the idle screw have the expected effect if you adjust it with the clutch disengaged (pedal up)?
Frankly, this one is baffling so I'll ramble a little in case it helps you or anyone else call to mind a potential cause.
The low clutch pedal could support the theory that there is excessive crankshaft end play. But if there's enough friction to slow the idle speed it seems like there would be noticeable noise.
Since there is no switch on the clutch pedal it seems unlikely that the problem is electrical, even though it sounds as though it could easily be electrical in nature (that's the first place my mind went). Even if the problem is a bad ground (the only thing I can think of in the absence of a pedal actuated switch) it's tough to see how a bad ground would lower the idle that much. In short (no pun intended), I'm having a difficult time coming up with an electrical problem that would account for the idle drop without revealing itself elsewhere.
Assuming it's not electrical in origin, what are the potential mechanical causes? Is there any way that the idle is actually controlled by the throttle linkage itself, and that depressing the clutch causes enough of a shift in the throttle linkage to drop the idle? Highly unlikely, since the clutch and brake pedals are totally separate from the throttle pedal, so for that to even be a possibility the throttle linkage at the carb would have to be really goofed up. But at least something to consider... If the car had mechanical throttle linkage I'd be asking about the motor mounts, but with a cable actuated throttle it seems unlikely that the engine shifting positions could cause the problem.
I can't think of anything else that could happen to the carb itself to cause the idle to drop that much. Can anyone else?
The only other mechanical cause my tiny l'il brame can conceive of is friction. And the only thing that comes to mind there is trashed thrust bearings. It could explain the idle drop, as well as the low clutch takeup point.
If it was my car I'd want to get a rough idea of the crankshaft end play in order to see if there's an obvious problem. Unfortunately, I don't know of a good way to accurately measure crank end play with the engine and gearbox in place. But you could check it visually-- the factory spec for a new thrust bearing is .0021"-.0104" (.055 mm-.265 mm), so a missing or really badly worn thrust bearing would likely be quite noticeable.
So here's what I'd do: start the engine, depress the clutch pedal, shut the engine off, wait for it to come to a full stop, release the clutch pedal. That should load the crank towards the front of the motor. Then take a pry bar or long screwdriver (I'm talking the 3'+ he-man version, not an actual screwdriver) and pry against the front pulley, using the lower longitudinal frame rail as a fulcrum. The allowable end play is quite small, so if you notice much movement at all further investigation would be in order.
Sorry for prattling but hopefully the questions and suggestions will be of some use in advancing the diagnostic process.
///Mike
