A Mercury Service manual is a real good idea, as the procedure has some details.
1 Synch carbs to open at the same time, using adjustment on plastic levers connecting carb throttle plates.
2 Remove spark plugs 2-6 and put timing light on #1. Remove prop. Set throttle such that carbs are still closed, but ready to open.
3 jumper starter (ign on) and read timing on flywheel. Adjust primary pickup screw on throttle arm to read 0-9* ATDC, without moving throttle.
4 Advance throttle to max. Jumper starter and read timing at flywheel. Adjust to 21*BTDC using max spark advance screw, on throttle arm.
5 install spark plugs and start and warm motor on flusher or in tank. Adjust idle to 850RPM or so and set idle stop screw also on throttle arm.
6 adjust throttle cable to push the throttle closed against throttle stop with some preload.
7 adjust idle mixture screws on carbs in the water, with a prop, warm engine and in gear.
