Backfiring on engine braking reminds me of my childhood..... I bet the local bicycle enthusiasts sure hated my dad's '67 ford pickup when he would pop one in their ear. 

 I had that problem briefly this spring,check for junk in your carb.I run an autolite and apparently after years of sitting on shelf,even after a cleaning there was a fly in my crossover between bowls.Believe it or not part of a flies wing was under the needle causing it to be just rich enough to cause a back fire when decel and just flooding enough to get on my nerves.Took very little extra fuel to do this.

You say you have been playing with the timing. My car always backfires on decel when the timing gets retarded (actually, kind of neat since it reminds me of the typical hotrodder sound with glass packs running too much carb back in the 50s. Very nostalgic.).

6 VOLTS/POS. GRD. NW INDIANA

The eddy 500 cfm is jetted down for Colorado elevation 5000 ft. Fuel pressure is at 5psi. I had stock jets in there and it ran way better however I could smell the richness pretty bad. Edelbrock says I will be a tad rich with stock jets but not necessarily too rich. Currently it is jetted down to their specs for Co. I am thinking with my set up I need a 600cfm carb. OR maybe i have a loose headed flang or collector. I'll check them this evening and update.

I’ll go with the fuel mixture being on the rich side.  Too much ignition timing is also a possibility.  You didn’t mention which way you were going with the jetting but sounds like you simply need to back it back up from whiscruber direction you were going if simply wanting to get rid of the backfiring when decelerating.

Lorena, Texas (South of Waco)

Hey folks. I have a 59 with 292, mummert intake, isky cam, eddy 500 cfm carb and the original rebuilt t98 trans. Also headers with glasspacks. I have been going through lots of different carb jet and spring variations as well as timing to get the engine where I want it. I still have very low vacuum at idle. I am sure I have no leaks but with my cam I only get about 14 at idle. So anyways I have some backfiring ( not really bad ) when I am engine breaking. It never did this with other spring and jet combos. I guess my question is what would this be a symptom of? Thanks for the help.