Holley 1920 need help

You need a vacuum gauge. Harbor freight sells a cheap one that'll do you just fine.

You want to be hitting at least 17hg on the gauge...20hg is ideal. Anything below 17 and you'll start having idle problems.

To find vacuum leaks, spray carb cleaner around the base of the carb and where the intake runners meet the head. Anywhere where air can escape. If the engine idles faster when you spray carb in a certain spot, you know it's sucking air through there and you'll need to fix it.