Removing the air cleaner on an inboard engine, or the hood casing of an outboard motor; adjusting the needle valve; and setting the idle speed screws for optimal performance and testing are necessary marine carburetor adjustment steps. Have someone else operate the boat while you make the adjustments.
After removing the air cleaner, or hood casing, inspect the hoses and the hose clamps connecting them to the carburetor. The hoses should be pliable and free of cracks or leaks, with the hose clamps tight. Locate the needle valve mixture adjustment screw positioned on the side of the carburetor, it will have a slotted screw head.
Start the boat and have your operator bring it up to mid-range speed. If your engine has been running rough start by using a screwdriver to rotate the mixture adjustment screw one quarter turn clockwise increments to lean the mixture. Repeat as necessary until the engine runs smoothly.
Have the operator put the boat in neutral and reduce the throttle to idle speed. Find the idle speed adjustment located near the connection point of the throttle linkage and the carburetor. Adjust the idle speed with a screwdriver or wrench until the engine idles smoothly.
Have the operator run the boat at high speed; if the engine runs rough the mixture is too rich. Reduce the speed to slow, and turn the mixture adjustment screw one eighth of a turn clockwise. Repeat the test until the carburetor is adjusted for high speed operation.