Mikuni and Keihin are the two primary manufacturers of carburetors used in modern PWCs. While I have experience with both, most of my practical carburetion experience has been with Mikuni carbs on Kawasaki, Yamaha, and Polaris watercraft, both stock and modified. The majority of current Yamaha and Sea-Doo models, as well as most Polaris triples, use Mikuni carburetors, while all current Kawasaki models (except the '96 750 SXi) and many new twin-cylinder Polaris models use Keihin CDK-II.

To help you understand the tuning of these diaphragm-type carburetors, I'll break it down into four basic categories: external adjustments, internal adjustments, jetting, and pop-off pressure.

EXTERNAL CARBURETOR ADJUSTMENTS

Idle Adjustment Screw
Under your flame arrestor, you'll see three adjuster screws: idle adjustment screw, idle mixture screw (low-speed screw), and high-speed adjustment screw. Both Mikuni and Keihin carburetors have these three adjustment screws. The idle adjustment screw is used to set the idle speed of your engine by opening the throttle plate (butterfly) and allowing air to pass through the carburetor. This screw is typically located near where the throttle cable attaches to the carburetor. On dual- or triple-carb models, there will only be one idle adjustment screw for the set of carbs. Turning the idle adjustment screw clockwise, or in, will increase the idle speed of the engine, while turning the screw counterclockwise, or out, will decrease the idle speed of the engine. The idle adjustment screw's goal is to achieve a smooth idle around 1000-1200 rpm while the PWC is in the water.

Idle Mixture Screw (Low-Speed Adjustment Screw)
The idle mixture screw, also known as the low-speed adjustment screw, affects the low speed of the engine, usually from idle to about one-quarter throttle. However, it also contributes to the total fuel supply all the way to full throttle. The low-speed adjustment screw works in conjunction with the low-speed jet. Carburetor manufacturers make it easy to identify which screw is the low-speed and which is the high-speed adjuster. The low-speed screw is the lower of the two screws on your carb, while the higher of the two screws is the high-speed adjuster screw. Both screws run parallel with the throttle shaft, and their location on the carburetor may vary by model.

With the engine not running, slowly turn the low-speed adjuster screw in until it lightly bottoms out, being sure to count the screw's revolutions. Repeat the process for the high-speed screw, but be cautious not to turn the screws in too far and damage the carburetor or screw. If you have a service manual, compare your adjuster settings to the factory specifications. If your adjustments differ by more than a half turn from the stock boat, adjust the screws to the factory specs. Otherwise, adjust both screws back out to their original settings, which you should have recorded in case things get worse during the adjustment.
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If your PWC has a rough idle, start the engine and turn the idle adjustment screw until it reaches a smooth, steady idle. Gradually open the throttle from idle to about half open. If the engine "gasps" and nearly dies, it is lean. If the engine "chugs" or runs like it does when the choke is on or after it has been run slowly through a no-wake zone, it is rich. If it revs up cleanly, it is in the right ballpark. To adjust the settings, turn the low-speed adjustment screw one-eighth of a turn at a time. A little goes a long way,