What most of those vacuum gauge tutorials omit is that at idle you might see a 1-2in drop on the guage. Its worse with a 4 cylinder. I was chasing the needle on my L134 in the jeep and it turns out that, at idle, a 4 cylinder will bounce 1-2 points.. is a valve timing issue. 6's and 8s don't bounce much, if at all at idle.
