I think the 4ac has a tendency to stall from factory.
I remember my dad talking about it one-day when my 4ac was doing it. Never could source/fix the problem.
Just turned the idle up to 1100rpm and it went away.
Ive seen it in 3 4ac now, even one which had just been rebuild from top to bottom.
Hope you find the problem cause i never did.
Perhaps someone on the forum might be-able to shed some light on the subject.