I'll assume your motor is an '86 model (correct this, or post a model number). The knob and needle on top adjusts the low speed air/fuel mix. Pull the plastic knob off. By hand, turn the needle clockwise until it is lightly seated. Turn it counter clockwise 1 1/2 turns, and push the knob back on. Start the engine and let it warm up. If not backfiring from the port on the leg, adjust the needle clockwise (leaner) 1/8 turn at a time until the motor backfires again, then back out the needle about half a turn richer -- should be close to the proper adjustment.
