Unsteady vacuum Readings

Hello there I have just put in a rebuilt big block in my mud drag pickup, and I had to reset the valves after the break in period cause I had weak valve springs in for the first half hour of run time. Anyways I was wandering how come my vacuum reading bounces around from 13- 15 inches at idle. I have to pull 15 inches of vacuum for tech in. I passed the first tech in. I was wandering if I don't have the valves set right. or something. No vacuum leaks around carb or brake booster that I can tell. Just need some suggestions. When I set the valves I just turned the motor over until I thought the pushrod was done moving down then tighted up while turning the pushrod when I felt resistance I went another 3/4 turn. First time I set valves so tell me if I'm wrong. Thanks

Loosen the pushrods with the engine running one at a time. tighten them until they atop making noise and go 1/2 turn farther. Recheck the vacuum. The vacuum on a fresh motor will tend to be erratic for several hundred or as much as a few thousand miles depending on the motor.

I just want to say both these guys are on the money as far as possibilitys.i just finished a similar problem myself.was a manifold leak.very small at that.i run about 10" of vac at idle and would fluctuate around 3 " slowly.now with that fixed it runs at right around 10" at 1000 rpms with a +-1 " due tp the extreme cam.just a reminder dont miss the little things cause they will haubt you till the end then youll kick yourself lol.good luck