502 Ram Jet Idle Problems

[d.c.]

502 ram jet that refuses to time out. I have set the engine in base mode using the supplied GM tool which disables the electronic advance. Gm spec the base timing at 10 degrees atdc. I can achieve around 6 to 7 before the engine dies. No matter where I go with the base timing it runs rough and will occasionally backfire. Firing order is correct and no trouble codes are present. I know the engine is still in green mode, but the rough idle seems too extreme for this to be the cause. Any ideas?