[zimmy]

A locked timing may be okay for racing where you are mostly wide open but I think that you would want variable timing for the street. I know Webers like a lot of timing. I'm not sure about your engine, but for my stroked 427 FE I run 17 deg initial with 21 degrees advance for a total of 38 degrees. The advance comes all in by 2800 rpm. I believe that this will help your popping issue as well.

I have been told that exhaust popping comes from an exhaust leak for sure, but that it is also worse when leaner than when rich. If you are going lean when you decel this will make the problem more pronounced.

A lean condition makes for a very poor wave front in the combustion chamber causing an inefficient burn and a fair amount of uburned gas to be left over. This unbured gas hits your hot headers and expoldes causing the pops. Your advanced timing when on the idle circuit (under hard decel with your foot off the pedal) will make the unburned gas issue even more pronounced.