As a first step, ensure that the ignition system is in top condition. Correct spark plugs w/ correct gap, clean distributor cap (inside & out), clean rotor.

Then, note the notch in the left side of the fuel distributor; in the notch is a "tower", open to the top. That it is open is a clue; it was plugged when new. In the tower is an adjustment for the mixture. Insert a 3mm hex key, push down & turn slightly. You will feel the screw drop a bit farther and engage the actual adjusting screw. Clockwise is richer. No more than 1/8 of a turn at a time; keep track of turns. After each adjustment (with engine stopped) release the adjustment screw, and try a start.