Timing Curve on 360 in an RV

Yes I forgot we were talking about an RV engine here. As suggested above you may want to curtail the initial timing to something closer to what the FSM says but you still should be able to run more than 21-22* total. The reason I suggested trying a higher all in number was to keep all timing from being in to early since you described the backfire issues. This has helped me avoid those issues.