Well i've noticed this started during the last hot laps of an auto-x event, and it is much more prominent and frequent when I'm open header. Say for instance i'm in gear (doesn't matter which, just any) i can floor it to redline without any hiccups (sounds and pulls nicely), BUT if i get off it completely and let RPM's decelerate on their own right around 3K rpm i back fire. So I'm stuck trying to guess what kind of threshold i'm crossing. I think my MSD ditributor timing might have something to do with it, as i have it set to hit full advance at 3K rpm. But I am unsure as to why it only occurs on deceleration and no acceleration (since there's more of a load during accel.). So I opened and cleaned my carb. and pulled the vacuum advance line from the carb to the distributor and it still does it.. 
...I wonder if perhaps the motor is experiencing a lean moment?? If anyone's followed what i've done recently (new afr's and COMPLETE msd system), maybe they can guide me towards checking other key dynamics I am not observing. OH FYI: Directly after my headers, there is an H-pipe connection which i thought would equal out the exhaust pulses and eliminate possible after-fires.....guess not.
Any help would be greatly appreciated!
...I wonder if perhaps the motor is experiencing a lean moment?? If anyone's followed what i've done recently (new afr's and COMPLETE msd system), maybe they can guide me towards checking other key dynamics I am not observing. OH FYI: Directly after my headers, there is an H-pipe connection which i thought would equal out the exhaust pulses and eliminate possible after-fires.....guess not. Any help would be greatly appreciated!
And yes, it should be lean, if the carb's butterflies are closed. Almost all my engines have experienced popping in the exhaust on decelleration. Even stock motors. Had a 69 Ford van with a 302, after I put duals and glasspacks, that thing sounded awesome coming down off a mountain pass with the trans in second and just the engine's compression providing the braking effort to keep the speed under control, 
my old man loved it too. He never critisized my dual exhaust afterwards. 
