Timing

Set timing last night 3400 at 2500rpm,ran great at idling in garage. Hooked vacuum back up took out for spin, started popping at about 2000rpm. Back in garage scratching my head, didn't make sense. Wife was holding idle at 2500 no problem, then I realized only thing different was vacuum was unhooked when timing. Today I unhooked vacuum started it up and took it up past 3500rpm no problem. Hooked vacuum back up to carb tried again, it started popping at about 2000rpm. So is it a bad vacuum advance module? would this cause the problem if not working correct?