A '73 Merc 850 should be battery-powered CDI ignition. It should have hot spark, regardless of cranking RPM, however, the choke requires good cranking speed to achieve a rich starting mixture.
The timing marks should be on the cowling support. Set the motor to TDC and remove the distributor pulley cover. The cast arrow on the distributor pulley should be pointing at the dot or dots on the timing decal. If not, slip the belt off and adjust.
Remove spark plugs 2,3,4 and install them into their wires, and ground their bases. Put a timing light on spark plug #1. Remove prop. Put motor in gear and advance the throttle until the carbs are ready to open, but still closed.
Now, jumper the starter solenoid, ignition on, and read the idle pickup spark timing. It should be 4-6* BTDC. If not, the brass-colored collar located about 1/2 up the distributor needs to be adjusted to that it is. There are two ss bolts with 5/16" heads which allow adjustment. Tighten them after you get the idle pickup timing correct.
Max spark advance s/b 21*BTDC. The stickers often say 23*BTDC, but gasoline was a bit better in 1973. Adjust the set screw above the distributor to set max spark advance, with the throttle wide open. Also set the throttle stop set screw to allow the carbs to be wide open, but not jammed against their stops.
Idle stop screw is set after carbs are adjusted to give no more than 1000RPM in neutral idle RPM.
Some other stuff to consider. At this age, the ignition wiring is often bad. The spark plug wires often arc to ground. The spark plug wires can have bad conductors as well. Triggers can be intermittent as well. Hopefully yours is good.
you have to push and hold the choke button.
