How to Adjust a Marine Carburetor

Many marine engine problems can be traced back to a few simple issues such as poor connections, a badly adjusted or dirty carburetor, or the need for a tuneup. A rough-running engine can cause extreme vibrations and also make an inboard engine's transmission sound as if it is failing. Yearly maintenance such as tuneups and oil changes help keep an engine running well, but some simple adjustments can make it easier to start, more reliable and much more efficient.

Things You'll Need

• Socket and wrench set
• Carburetor adjustment tool

Instructions

Adjust the Carburetor

1. Start and run the engine long enough to bring it up to normal operating temperature. Shut down the engine and open the engine compartment. Remove the flame arrester and any decorative covers to allow carburetor access.

2. Locate the idle adjustment and mixture adjustment screws. The idle adjustment screw will be on or near the throttle linkage, and the mixture adjustment screws will be near the carburetor's base.

3. Start the engine and place the throttle in the idle position. Ensure there is very little play or slack in the throttle cable where it meets the carburetor linkage to ensure the engine is truly at idle. Adjust the idle screw on the linkage to bring the engine's tachometer to 750 rotations per minute.

4. With the engine idling, turn the idle mixture adjustment screw clockwise slowly until there is a noticeable drop in the engine's rpm. You may need a special tool depending on the make or model of the carburetor. Turn the screw counterclockwise 1/2 turn. Slowly turn the screw counterclockwise 1/2 turn at a time, counting the number of half-turns until there is a noticeable increase in engine rpm. Turn the screw back clockwise 1/2 turn. Turn the screw clockwise half the distance between the rpm drop and increase. For example, if there were four half-turns from the drop to the increase, turn the screw clockwise two 1/2 turns, or one complete turn.

5. Adjust the engine idle adjustment screw on the linkage as needed to bring the tachometer back to 750 rpm. Reinstall the flame arrester and covers and test-run the boat, checking for smooth operation, clean exhaust and a full operating range on the tachometer.

Tips & Warnings

• Tune up your engine first to ensure rough conditions are not caused by spark plugs or worn distributor components.

• Set the engine timing correctly to ensure efficiency and performance.

• Use extreme caution when working around a running engine. Remove all jewelry, secure loose clothing, and keep your fingers away from belts and pulleys.

• Check the engine exhaust for black smoke as evidence of a rich running condition.

References

• Innovate Motorsports: Carburetor Central

Resources

• Carbcareusa.com: Replacement Carburetors and Parts