Idle vacuum - why only 10 inches?

[davepl]

My truck only makes about 10 inches of Hg for vacuum at idle. That's a fairly low idle of about 600. Pop it into gear and the idle falls to maybe 500, and the vacuum drops to about 7. Much more than that and you're into the power valve I assume!

It runs pretty well. It doesn't stall (barely dips at all) when the AC cuts in, and there's no kicker solenoid on these, so it has to manage without a bump in idle speed.

There are no leaks that I know of, and inducing a leak by unplugging a hose makes it run noticeably worse (which I don't think would happen if there were another large leak). Also it runs worse with the PCV blocked, so it wants that additional air. All that leads me to believe no leak, anyway.

But is 10" too low? What's normal for a stock truck with a very-close-to-stock cam in a 402?

Where would I start chasing to improve it? I tried more timing, without success, and mixture screws can worsen but not improve it.

A higher idle might help, but it's not like it's 16 at 1000. I'd like 15 at 750, but haven't had any luck getting it.

Like I said, it runs OK. If the gauge said 16 I'd pat myself on the back, in fact, but it doesn't, and I don't know why it's low. Maybe it'd run even better! Gauge is a large tune-up style one, quite accurate so far as I can tell and compare.