Q1,Are your exhaust pipes shared between sides?
I'm going with a bad header-flange gasket on the side that pops.
At closed throttle decelleration, the headers will pull a vacuum at the flange, and if the AFR is rich,or the ignition timing is late,the still burning gas mixes with the fresh air being sucked in by the header, and explodes, causing a pop in the pipe. If the exhaust was dropped you would probably see flames coming out of the collector.
This is more apparent under compression braking as the engine draws excessive amounts of fuel past the not-quite-closed-enough throttles.So that leads to the question,
Q2) what is your idle speed? and Q3) have you synced up your Transfer ports?What Champion heat range does your plug crossover to? I run Champion RN12YCs, IIRC, 12s are fairly cool. Your engine might like 9s.
So to recap; get your T-port synced up, hoping to set the idle closer to 650/700 in Neutral, Put the carb back to stock,and fix the header-flange leak. If you have an IR gun, you might be able to prove the leak. It will be the tube that runs the hottest about 2 to 3 inches from the flange. It will be at least 100 to 300 degrees hotter than the others. It runs hotter cuz the gas is burning in the pipe.
Good luck.
BTW
Once the T-port sync is established, do not mess with the speed screw! Don't be surprised if it takes less initial timing to slow the engine down. 15* is, IMO, too much for a cam that idles at 18/19 inches. I think 10 or 12 will be better. Getting the transfers synced at idle is job #1.
For help with this click on the little blue M below, or go visit the Holley site.
Also,IMO 35* at 2500 is too much . But if it doesn't detonate then I guess you are alright. I just know that I would not run that much. Then again,I run 87E10, in my 10.9compression 367cuber.
Q4) What stall are you running?
