Retarded timing when A/C on

[LarsB]

So, I have had idle speed issues lately on my '90 4runner V6. Low idle but usually when I turn on the A/C the rpms increase like they are supposed to even when the idle is around 500. About a week ago it started running rough. I changed the plugs and checked the timing (good @ -10°). After poking around I notice the rough idle is associated with the A/C and when I turn the A/C on the timing retards 5° to -15°. Does anyone know what would cause this? Thanks.