340 Idle problems

Vacumn advance has been unused since I received the car, wasn't even hooked up. I never thought much about it figuring whoever built this engine knew what they were doing, as it ran so well. Seat slamming acceleration with the 4.30's and has cruised effortlessly to numerous car shows (although it is a very noisy 340 with the 3" flowmasters and downturns). I thought it odd that the timing at "idle" was so far advanced, but who was I to mess with something that seemed to work. So I checked all the present settings, wrote them down, and went about the enjoyment of driving my Mopar. Only this "simple upgrade to some roller rockers" has brought to light a problem. Now that I think about it, while cruising at 50ish before this problem, I remember the exhaust always popped! Not a miss sort of pop, just a rather regular popping sound. By the way it looks to be running a stock mopar electronic distributor with the Crane ignition box and LX92 coil. Thanx, for the responses!!