Right in front of the throttle linkage, on the driver's side of the carb, is a solenoid. It has a light green wire attached to it. At least, it's light green on my '79 - I'd assume the '78 is the same.
Disconnect that wire, and see what happens. Make sure to tie the wire off to something, though, so it doesn't get caught on anything when the car is running.
I'd also suggest disconnecting the compressor clutch for the duration of your test. It's the electrical connector just behind the compressor pulley. With that connected, the compressor can't be activated, so you won't have any problem with the compressor kicking on and stalling your motor because the solenoid isn't speeding-up the idle.
If this fixes the problem, then you need to track down why your A/C is kicking on. The electrical diagram should be in your wiring diagrams, and isn't all that complex. Most likely, there is a loose connection or a short somewhere (or both), that's triggering the A/C. If you don't have a wiring diagram yet, let me know and I can scan in that section and email it to you.
You also need to adjust the solenoid's position - like Bob says, it shouldn't be increasing the idle that much. I'd suggest jumpering power to the solenoid connector to keep it engaged, then slowly adjusting it while someone watches the tach, until it is at the correct idle. I believe the number is on the emissions/tune-up sticker. It should be stuck to the top of your firewall, just under the hood, on the driver's side. At least, that's where I think mine was. It was pretty loose and came off when I washed the engine compartment.
Joe
