Outboard carburettors are designed and function similar to automobile carburettors. Both meter fuel in precise amounts to the engine for optimum internal combustion. Outboard carburettors have a power circuit, metering jets, float bowl and low speed and idle circuit. Adjusting outboard carburettors requires a knowledge of the position of the mixture and idle speed screws on the carburettor.
Submerge the lower end unit into a larger rubber container filled with water. Cover the water intake ports, but allow the exhaust port to sit above the water level.
You can also perform this procedure in the water by tying up securely to a dock with fore and aft lines. Check your owner's manual to see if your carburettor adjustment can be made in the water with a neutral transmission setting. Many models require that the engine be in forward gear, with the prop producing resistance in the water.
Unclasp the top engine cowl. Remove the spark arrester, or air cleaner system, with a socket if it impedes access to the carburettor. Start the engine and let it warm up to normal operating temperature. Deactivate the manual choke, or wait for the electric solenoid choke to open fully, if you have this model. Turn the engine off. Locate the idler mixtures screws for your carburettor. Refer to your owner's manual under "Air-fuel mixture setting."
Place a small engine tachometer clip lead around the number one cylinder spark plug. Connect the other tachometer lead to a ground source. If you have a dashboard tachometer gauge, have an assistant monitor it for rpm readings. Turn each idle-mixture screws in clockwise until they gently seat. Then turn them counterclockwise 1 1/4 to 1 1/2 revolutions. For a multi-cylinder engine with dual carburettors, adjust each idle mixture screw with this procedure.
Start the engine and place the shift selector in forward gear.Turn the idle mixture screw in clockwise until the engine sputters. Turn it out counterclockwise until the engine begins to miss. Turn it back in until you achieve the highest rpm on your tachometer, which will be a midpoint between the two stumbling screw positions. Adjust each mixture screw in this fashion, achieving the highest rpm on the tachometer gauge.
Locate the idle speed stop screw on the carburettor linkage. You will recognise it by pulling back on the throttle linkage and seeing where a small screw rest against a cam. The screw will have a throttle return spring very close to it. Look up the idle speed specifications for your outboard in your service manual. It will recommend a maximum idle speed for a warm engine.
Use a screwdriver to turn the idle speed stop screw clockwise to increase the rpm, and counterclockwise to decrease it. Set the rpm to manufacture's specifications, reading that number on the tachometer. Turn the engine off. Remove the tachometer. Replace the spark arrester, or air cleaner assembly and tighten it with the appropriate socket. Fasten the engine cowl.
Some model outboard motors have an high-speed idle screw. Others do not have one because the jets are preset. Consult your manual for the proper rpm to set your high-speed idle screw. Use a screwdriver and a small engine tachometer.