HEI ignition

I just got my Classic HEI ignition running today. Ran great at first, I was checking timing, pulled off vacuum line suddenly it ran like crap. Hooked vacuum line back up and didn't help, then suddenly it smoothed out then back to running bad. Gave it some gas, at about 2500 to 3000rpm it started popping then went away when let off gas. I have a call in to Trailbeast but thought I would throw out here if anyone had any ideas if I missed something. One thing I noticed is when I pulled vacuum line off carb it did't change timing at all. Bad vacuum can maybe? thanks