Re: Direct Charge or "Plain Vanilla" crossfire?
The 650 was Old School and did not have the high, curved deflector on the pistons, as were present on the 99-c.i. Sixes and the 4-cyl 66-c.i. 800 850, etc.
So I reckon you could call that "Plain Vanilla Crossflow"! Or what Merc used to advertise as "Power Dome". Sounds powerful, don't it!
The timing spec for your 650 is 38 deg BTDC for full spark advance, and the older motors aren't as sensitive to timing or as prone to preignition as the "deflector" motors.
So, if you are feeling somewhat conservative, you might dial back full advance to 36 deg or so, but this motor probably won't care much if you left the spark advance at the factory spec.
However, if you were to set it back to the "deflector" motor recommendation of 18 deg BTDC, it won't run well at all!!!
Mercury Marine 650 (4 CYL.) CRANKSHAFT, PISTON AND CONNECTING ROD Parts
HTH...........ed
