JetSki Carburetor Fundamentals

Making adjustments to the Jetski carburetor air/fuel  mix is pretty easy. nearly all watercraft carburetors are fitted with high speed (30-100 percent throttle) and low speed 90-30 percent throttle fuel mix adjustment screws.  In a stock application the screw closest to the mounting plate of the carb is always the low speed screw; The one nearest the air intake is the high speed adjuster. turning them out (from bottom closed position) will richin the fuel mix.

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Pop Off Pressure

The relationship between pop-off pressure and low speed jets. " those two circuits overlap" although the low-speed jet continues past 1/4 throttle engine operation, whereas pop-off pressure has little to no effect due to increasing air velocity in the carb. also, one can make up for slight inadequacies in the other.
"For example, if your pop-off pressure is a bit high, you can compensate by increasing the size of the low speed jet, The opposite is true, too: a slightly undersized low speed jet can be compensated for by using less pop-off pressure."

When does it become necessary to adjust the pop-off?
"Pop-off pressure refers to the pressurization of the needle valve with compressed air, through the fuel inlet," "this is a reference to the amount of pressure needed to open the needle valve-- or 'pop' it off its seat. The higher the pop off the more pressure required.

"When personal watercraft or Jetboats come from the factory they have fairly high pop-off, due to the fact that they also have somewhat restrictive air intake systems that cause the engine to generate very high manifold pressures. The higher the manifold pressures, the higher the pop off pressure required to properly regulate fuel delivery to the engine"

As you modify or change your flame arresters to a less re-strictive type ( meaning more flow), you are lowering this manifold pressure--- and are inadvertently creating a 'lean' condition in your engine. Changing the pop-off pressure--- which is nothing more then switching the Needle Valve Arm Spring in the regulator to a spring with a softer compression (gram) rating. "sometimes in order to achieve the desired pop-off pressure it's necessary to also change the needle valve size. Just remember its always better to use the smallest needle valve size to obtain the correct pop-off"
​Tools Use Pop-Off Gage

Carb Performance Trick

Remove the choke and replace its function with a primer kit. Though most PWC enthusiasts tend to regard the use of a carburetor primer as "band aid" for an ill tempered engine or comp device, it offers benefits for virtually any watercraft.The single biggest limitations to a carburetor operation is airflow; anything you can do to improve the flow (and velocity) of air through the carb will usually result in increased performance.  Choke assemblies, which incorporate regulatory devices similar in appearance to throttle plates (but located at the top of the carb) are an obvious impediment to air flow.

Basic Mikuni Super BN Calibration

There have been a number of diffrent carburetors used to for feeding PWC over the years, primarily by mikuni, with the end result being the popular mikuni super BN carb. It is designed specificaly for watercraft -- meaning it is essentially a sealed unit that canot vent its fuel chamber to the atmosphere (which would allow fuel to leak into the hull in rollover situations).
 Though accurately tuning this (or any) carb for your application is limited by other possible weal linmks in the engine,  Use this procedure for calibrating tune-able circuits during testing - 
1 - Low Speed Adjusters: to adjust for smooth idle
2 - Pop-Off Pressure: just off idle to 1/4 throttle  ( in conjunction with low-speed jet).
3 - Low Speed Jet: just off-idleto 1/3 throttle
4 -  High-Speed Jet: 1/3 -to 3/4 throttle
5 - High-Speed Adjusters: 3/4 to wide open throttle