How do I correctly adjust the air-mixture screws on a marine carburettor?
My petrol boat engine runs roughly at idle, and I want to reset the carburettor mixture screws. Do I turn them fully clockwise and then back them out, and how many turns should I use?
Answer
If they are adjustable idle-mixture screws, turn them clockwise until they lightly seat, then back them out to the starting position specified for that carburettor. Never tighten them hard, as this can damage the tapered tips and seats; make the final adjustment with the engine fully warm.
Identify the mixture screws
Check the carburettor make, identification number and engine model before turning a screw. A screw that meters fuel works in the opposite direction to one that meters air. The engine year alone can mislead if someone has fitted a replacement carburettor.
Common petrol inboards use Rochester, Mercarb, Holley or Carter/Weber carburettors. Depending on the model, there may be one idle-mixture screw or a matched pair; some performance carburettors have four. Outboard arrangements vary considerably, and some have fixed jets with no routine mixture adjustment.
- Fuel-mixture screw: clockwise usually reduces fuel and makes the idle leaner; anticlockwise usually makes it richer.
- Air screw: clockwise usually reduces air and makes the mixture richer; anticlockwise usually makes it leaner.
- Idle-speed screw: bears against the throttle linkage or stop and changes throttle opening rather than directly adjusting mixture.
Screw position offers clues, but confirm its function in the correct manual. Do not use a generic idle-setting procedure to adjust sealed settings, throttle synchronisation screws or high-speed needles.
Set the starting position
With the engine stopped, clean around the screws so dirt cannot fall into their passages. Use a screwdriver that fits the slots properly.
- Record each existing setting by counting clockwise turns to a light seat, including fractions such as a quarter-turn.
- Stop as soon as you feel contact. There is no tightening torque to apply.
- Back each screw out by the manufacturer's specified number of turns.
About 1½ turns out is a common starting setting on many conventional inboard carburettors, but it is not universal or a finished adjustment. Other designs require different settings, particularly outboards. If the engine previously ran reasonably well and you lack the specification, return to the recorded positions rather than guess.
On a conventional two-screw carburettor, begin with both screws equally open unless the manual says otherwise. Small differences after tuning can be normal.
Adjust with the engine warm
Before starting, check for petrol leaks and ventilate the engine compartment. Operate any fitted bilge blower as instructed, provide cooling water and keep people clear of belts and the propeller.