I installed a new cam in my car the other day (along with a new torque converter) and broke it in properly (15-20 minutes at rpms ranging from 2500 to 4000 rpm). It took a little while to get it timed right (the rough timing was too far advanced) but my friend finally set it at 34 degrees (fully in) while I broke in the cam. I drove the car around the block a few times, and everything seemed great (not a single backfire). I started it later that night to put it into the garage, and had no problem.
Three days later (today) I started it, and it began backfiring through the exhaust immediately while at idle. I advanced the timing a little (I don't have a digital timing gun, so I don't know exactly how much...probably a degree or two) and it is still backfiring through the exhaust. This scares me because last time I had a backfire problem, my starter kept eating my ring gear teeth because the backfires had caused my cam timing to jump (see this post: https://board.moparts.org/ubbthreads/showthreaded.php?Cat=0&Number=5316166&page=).
34 degrees fully advanced seems a little retarded for my combo. The specs for the engine are as follows: stock 440 block (.055” over), stock forged crank, no-name aftermarket H-beam rods (w/ 7/16” bolts), KB forged pistons, Indy SR OOTB (gasket port), Indy 2D Intake, 850 cfm Holley DP, 440 Source roller rockers, TTI 2 1/8” headers & 3" exhaust, Pro Comp dist., 7 qt. pan, .545"/.545” lift Comp hydraulic flat-tappet cam. The compression ratio is about 11.5:1 and I run a race gas mixture.
It seems to only be backfiring through the driver's side exhaust. So any ideas about what's going on? Timing? Flat cam? Loose spark wire?

Thanks in advance for any help.

Sounds like you need to find a different friend since you have had backfiring issues before j/k!

Right off I would say you have a bad plug wire, bad plug, loose plug, broken plug.....you can see where I am going.

If you are way off with your timing that can be a problem but since it seems to come from one side I would start with the a wire or a plug.

I would look to plugs and wires. maybe fouls a bit when you start it. mine does that then clears up?? so I ignore it!!
you might be hearing ghosts!! when you nervous you hear them more! LOL!!
Loud backfires or little tinging pops?