The air intake is for the COOLING FAN. Don't run the engine without the COOLING FAN air duct working. You will burn it up.

If I remember right, the screws on those carbs turn IN for richer, OUT for leaner. Get a baseline setting (1 1/2 turns from all the way in) and go from there.

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If it ain't broke, and you mess with it long enough, it will be.