fuel mixture sitting different for AC vs no AC

[jwakil]

I was trying to slightly adjust my fuel mixture on my 77 3.0 CIS engine. What I realized is that the optimized setting is somewhat different if I'm using the air conditioner or not. If I had the AC off, I would notice that when I revved high and lifted off, I would get some minor backfiring, suggesting lean. So I would richen it up slightly until I got no backfire. However, when I would have the AC on and revved and then lifted my foot off the gas, the rpms would drop way too much almost to stall (suggesting too rich). So it seems like you can optimize for with or without AC load, but not both. Is this normal or might I have other issues?

[schoward]

It can be achieved.

I had this issue a few years back on my SC where rpms would drop too quick and cause stall when AC was on but not when AC was off.

My wrench fiddled with mixture and timing and kind of got it better. I continued the process with mixture and idle speed changes and has been great for 3-4 years now.

I run CO around 3.0 - 3.2 and adjust warm idle speed to be 900-950 (AC OFF). Not sure what my timing is set at but pretty sure its stock setting. The invoice from my wrench back then simply said "tried multiple combinations of timing and mixture to get rpms to drop slow at an acceptable rate" or something like that.

So - it can be done. My car is fine now even when bone cold and hit the AC on right out of the driveway. I get a little bounce down when cold but nothing that comes close to a stall. I do not run with decel valve connected.

Frustrating I know. Do some searches here if you havent - I know others have had a similar issue - may give some more ideas to try.

[jwakil]

Cold is fine since idle is faster anyway, when it gets hot is when the problem comes into play. I think I need to keep it slightly lean in the summer when I'm using my AC, and richen it up a little for winter to avoid the sputtering.