Not a Mechanic but it seems no one else is posting so Ill throw you a bone. How old is your compressor and is it it good shape? Cuz best I c an think of would be the compressor. Most likely im wrong though however thats the first thing Id look at. Also when you turn on the ac and your under the hood, does anything start making a out of the ordinary sound?
Ya im having that problem on my 2000 rolla:hammer:
U turn the car on and a/c is switched to the on postion
when u push on the gas the car barely moves it struggles to a crawl. But once U turn off the A/C the car goes.
The Idle-up valve (IUV) is not just to spike the idle... it also controls it when the a/c is on... if it is broken, it will flicker on and off (or not at all) but will still spike the idle to start with, confusing the compressor, MAP sensor (or AFM) and ECU (if any).
These mixed signals will cause the rough idle that you talk about. When the Throttle Position Sensor (TPS) is open far enough, the IUV switches off as there is no need for the extra Revs to power the compressor. The fact that it idles fine in park is just that there is more load on the engine in Drive, therefore adding to the stresses of the mixed signals from the IUV.
Hope this helps, and if you have any further questions, just ask...
I'm not too sure what its called on your model, but its there. It will be a small box with vacum hoses that run from the pleunem to the valve, then from the valve to the intake tubing.
All this does is create a loop in the air flow, directly impacting the amount of air flowing behind the semi-closed butterfly(s), thus lifting the idle. The fact that it is carb has no real impact on the a/c as even on EFI controlled cars, the vacum hose bypasses the butterfly(s).
Sorry i cant be of more help as to what it is called, but i specialize in the electronic side of things.
If the rpms on a four cylinder drop too low it will get really rough. The more cylinders you have, the smoother your idle. My 91 idles a bit rough, but when i turn on the a/c, it idles up and there is no roughness. See if your idle is actaully increasing when u turn on the a/c. If its not then do what everyone else was suggesting. If it is, then I don't know what to say. It shouldn't be rough at higher rpms off idle.