[chopper]

Turk, I don't know if this is going to be of any help to you, but when I went through this, I ended up with primary jets at 60. Original jets were 64s, I believe. However, if you're backfiring on deceleration, it's more likely a problem with leaks in the exhaust system than jetting problems with the carb. The ERA cars use the "slip fit" (or "slip leak", which is more accurate) pipes and they can be a bugger to get sealed. I have Stainless Specialties sidemounts, and both SS and ERA had little useful advice during the trials when I was trying to eliminate the backfire which appeared when I installed the SS pipes. I tried different sealants and such and finally had the pipes removed and individually sized with a little pipe expander gizmo which I found at a flea market. We got almost all of the backfire problem resolved, although I still get an occasional "pop" out of the right exhaust.

I am running 600cfm 4160s with vacuum secondaries, MSD 7AL box with billet distributor, 10:1 compression and have both of my carbs set up with identical primary jets. The secondaries, as I'm sure you know, are not adjustable since that function is taken care of in the secondary metering plate. IMHO, if the plugs are showing the correct color, you're probably running close to the correct jet size. I suggest that you very carefully check for exhaust leaks while the engine is idling to see if you have some leakage around either the header/head mating or the header/sidemount joint (more likely the latter).