I've a very similar problem with my '90 4.2, but involves the vaccum tube at the rear. When cold, runs fine - except I'm in the Cayman Islands, cold doesnt last long.
Periodically it'll stall at idle, or downshifting esp without the a/c on. I notice if I turn on the a/c then the idle speed increases a couple of hundred or so, not huge but sometimes then idles over 1000 so too high. There's some kind of load sense which increases power by throttling up slightly (the accelerator sinks). Perodically it'll drop down (dont think it's sticking as I've tried blipping the throttle nothing happens, then all of a sudden it drops down, only to increase a few mins later :S
So I started to check for vaccum leaks and noticed at first one tube just hanging at the rear of the carb
- I traced this to the front of the air intake (before filter, manual says "reverse delay valve" what is that??).
Second and with the engine almost dying, I noticed another tube exiting the rear of the carb, going into a small diaphragm with a moving needle behind. When I removed this, the needle sat back, the engine idle jumped back to normal and the carb sucked in air thru this, but all seemed to work fine.
What (aside from unfiltered air in the carb) am I damaging by running like this? The engine has been idling perfectly (only ran upto 50mph)
I'm thinking of doing the nutter bypass as I suspect some shenannigans with the computer etc, but any ideas welcomed on that vaccum pipe?