Does it have battery powered Distributor CDI ignition? A 1970 should.
You should get an OEM service Manual. Here is the timing procedure.
First remove the timing belt from the distributor pulley. Spin the pulley and make sure she is not binding. A binding is likely can break the belt. Turn flywheel and distributor pulley until cast arrow on distributor pulley points to dot (or three dots) on flywheel timing decal. Install timing belt.
Set idle mixture screw carbs to 1-1/2 turns open, on all three carbs. Remove spark plugs 2-6 and ground wires to block. Put timing light on #1 spark plug. Crank motor and adjust throttle until she reads 4-6* BTDC.
Put 5/16" wrench and loosen two vertical bolts on distributor body. These hold a brass collar. With bolts loose, turn collar until finger just touches carb operating lever. Make sure to take up any play in carb lever, so if distributor moves any more, carbs will start to open. Tighten bolts.
Advance throttle full and crank motor. Loosen or tighten max spark advance set screw (on plate above distributor), until max spark advance is 21*BTDC. Tighten set screw nut.
Install spark plugs and run her on flusher for test. You must launch boat and adjust idle mixture on water, motor warm and in forward gear. Adjust all idle mixture screws to achieve smoothest, fastest idle. Now test motor for acceleration. If she stumbles, open top idle mixture screw 1/8 turn and test acceleration again. Repeat with mid carb idle mixture screw, and finally bottom carb idle mixture screw, testing after each adjustment. Repeat as desired,until idle quality is about 1000 in neutral and reasonable, and motor accelerates smoothly.
