Sven-
Popping could be excess unburned fuel in the exhaust going off, when decelerating, or an exhaust leak. Make sure there are no leaks in the exhaust to rule out that cause. Engine vacum affects the situation also when decelerating, a lean fuel situation occurs, some unburned fuel makes it into the exhaust and may cause popping. You may be able to decrease the popping by playing with the air/fuel mix or timing. I have a 1 barrel Rochester and the lowest flow electric pump NAPA sells. I get a little popping, I'd call it a soft burble, on deceleration. In fact I sometimes feel the engine idle drop significantly after a long deceleration and I finally push the clutch pedal in to come to a stop. I don't seem to be able to totally tune this out of my truck. It doesn't seem to be a problem at this point and I haven't had any "backfiring" since I removed the bad piping and muffler and replaced it with a generic cheapo muffler and pieces from a local parts house. I've timed my truck where it's running good I then have leaned the air/fuel mix for the best idle and throttle response. At this point I live with the burble and call it good.
Jeff
1964 Series 2A SW, LHD mostly stock, often runs!
1991 Range Rover Hunter