[bnoon]

Good news! Turns out it was just a totally backed off exhaust rocker on cylinder 3 causing a HUGE backfire through the carb. So loud it was making the metal on metal sound I was hearing. I used a file to cut some teeth into the bottom of allen set screw, adjusted lash, reseated the pushrod (after checking it was straight and checking surfaces on the rocker) and reassembled. VROOOOOOMBA!!!!