How to Adjust Boat Carburetors

by William Machin
Boat carburetors are adjusted with the engine running.

Boat carburetors are adjusted with the engine running.

Like most carburetors, boat carburetors are designed to allow a number of adjustments without disassembling the unit. Throttle linkage settings are key to maximizing the power of the engine. Needle valves and idle settings are essential to smooth operation. These adjustments are not difficult and in most instances, fine-tuning from time to time maintains peak performance. You can adjust an inboard or outboard boat carburetor in a short time with the help of an assistant.

Remove the air cleaner on an inboard engine and set it aside. Remove the motor case on an outboard motor and set it aside. Inspect any vacuum hoses to make sure they aren't cracked. Vacuum hoses should be pliable and connected to a boat carburetor with small hose clamps. Nylon ties do not provide an airtight connection. Remove any nylon ties and install hose clamps before proceeding with the carburetor adjustment.

Check the throttle linkage by operating the throttle lever by hand. The throttle cable attaches to the linkage; rotating the throttle lever away from the cable opens the throttle. There should be no play in the linkage. Use an adjustable wrench and turn the adjustment nut on the cable clockwise to take up any play.

Locate the needle valve adjustment screw on the side of an inboard or outboard carburetor. The adjustment screw has a screwdriver slot and a small spring that seats below the head of the screw. Proceed to the next step before making any adjustments.

Start the engine and get the boat under way. Have an assistant drive the boat at a medium rate of speed. If the engine has been running rough, turn the needle valve adjustment screw clockwise in 1/4 turn increments until the engine runs smoothly.

Have your assistant put the boat in neutral and throttle back to idle. Locate the idle adjustment on the side of an inboard carburetor. The adjustment is in close proximity to the needle valve adjustment screw and may have a hexagonal head. The idle adjustment is on the underside of an outboard carburetor.

Use a small adjustable wrench or screwdriver and rotate the idle adjustment 1/4 turn clockwise until the engine idle slows. Rotate the valve counterclockwise in 1/8-turn increments until the engine idles smoothly. Ask your assistant to put the boat in gear and run it at a high rate of speed. If the engine runs rough at high speed, the fuel mixture is rich and the needle valve needs additional adjustment. Proceed to the next step for this adjustment.

Begin the final needle valve adjustment with the boat running at a slow speed. Use the screwdriver and turn the needle valve screw clockwise 1/8 turn to lean the fuel mixture. Run the boat at high speed and repeat the procedure if necessary until the carburetor is adjusted for high speeds.

Stop the boat. Replace the engine cover on an outboard engine. Reattach the air cleaner on an inboard engine.

Items you will need

  • Vacuum hoses (optional)
  • Hose clamps (optional)
  • Screwdriver
  • Adjustable wrench
  • Assistant

Tip

  • You may need to rebuild the carburetor if typical adjustments don't resolve problems.

About the Author

William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.

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