This is weird. I got a pro billet msd, new from someone never used. Popped it in, and backfired at idle. Swap in the original one that I locked out and runs fine. I wanted to go from the locked out one to a mechanical advance. Even if I lock out the msd it backfires. I’m running a pertronix hp digital box, basically same as an msd 6al and a blaster 2 coil. Checked the gap and it was 20 thousands on the air gap. Anyone ever run into this?