How to Adjust High & Low Jets on a Jet Ski Carburetor

by Chris Stevenson

Most personal watercraft, such as Jet Skis or Sea-Doos, have carburetors that meter and atomize the fuel for combustion. An out-of-adjustment carburetor can cause misfiring, hesitation, stumbling or overall poor performance. The low- and high-speed jets on a personal watercraft determine the amount of fuel and air that enters the cylinder for combustion at different speeds. A too-lean mixture will cause starvation and hesitation; a too-rich mixture will load up the plugs and cause sporadic misfiring or flooding. Most owners can synchronize the low- and high-speed jets using the service manual and basic tools.

Mount your personal watercraft on a trailer and take it to a convenient lake for proper test running. Before you launch the craft, open the engine hood by unfastening the snaps. Locate the low- and high-speed adjustment screws on the carburetor. Refer to your owner's manual for their location. The low-speed screw might have a small T-handle and sit lower on the carburetor. The high-speed screw might have a plastic cap; if so, remove it.

Start the engine and let it idle for five minutes or until it reaches normal operating temperature. Your engine idle should be within with your manufacturer's recommended setting, somewhere around 1,100 rpm. To adjust the idle up or down, locate the idle stop speed on the carburetor linkage and use a screwdriver to lower it (counterclockwise) or raise it (clockwise) to within specifications.

Shut the engine off. Use a socket and wrench to remove the old plugs from the head. Install new, properly gapped spark plugs into the head and tighten them with a socket. Run your watercraft on calm water at no more than quarter-throttle for five minutes. Shut the engine off or pull the ignition key lanyard.

Remove the spark plugs with a socket and examine the end rims of the threaded part on the plug; they should appear medium to dark brown. If the plugs appear black, use a screwdriver to adjust your low-speed jet screw counterclockwise by 1/8 turn. If the electrodes appear white or cream-colored, adjust the jet screw clockwise by 1/8 turn to enrich the mixture.

Clean the new plugs with alcohol and a small wire brush. Reinsert them into the head. Tighten with a socket. Start the engine and run your watercraft at half-throttle for five minutes. Shut the engine off or pull the lanyard. Use a socket to remove the spark plugs and examine them for coloration. Use a screwdriver to adjust the high-speed jet screw clockwise by 1/8 turn if the plugs appear black. Turn the high-speed jet screw counterclockwise by 1/8 turn if the plugs have a white or cream color.

Clean the spark plugs with alcohol and wire brush. Reinsert them into the head. Tighten with a socket. Run your watercraft at 3/4 throttle for two minutes, then at wide-open throttle. If the throttle hesitates and the engine rpm does not pick up right away, you have a slightly rich condition. Give the high-speed throttle jet one additional 1/8 turn counterclockwise with the screwdriver.

Run your watercraft at 3/4 throttle or slightly more for two to three minutes. Cut the engine or pull the lanyard. Remove the spark plugs with a socket and check the coloration. If the color appears too white or black, locate your high-speed adjuster screw, if you have one, by referring to your owner's service manual.

Turn the screw clockwise if the color appears black. Turn it counterclockwise if the color appears white or creamy. Turn the screw in 1/8-turn increments, just like you did for the low- and high-speed jets.

Items you will need

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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