86 coupe a/c belt

[DK]

I had a slightly similar issue with my 4000. The belt was new and it
was tight but it wouldn't stop squealing at idle after I changed the
timingbelt. It would go if I would rev the engine, but before it would
get really loud then the squeal would disappear, but came back at
idle. Later I realized that my timing was off and the engine was
idling way too low.
After I adjusted the timing and idle, everything was back to normal.
This may or may not apply to you...

Dave