Large rpm drop

Ruptured PV diaphragm from a backfire..?? Never had that happen myself, but supposedly a common issue for these PV's after a good backfire.

I don't understand needing 30* advance for this latest idle number, with this cam and this SCR/DCR, and the secondary slots closed off. Something sounds fishy in the timing advance measurements....??? Or just super rich with the PV problem....