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Super BN


Accurately tuning a Super BN, for any application, requires a

basic understanding of its function and adhering to a few basic rules.

Most importantly, you can only expect the carb to work as well as your

engine does; the performance of your Super BN cannot make up for a

weak or worn out engine.

Another point to stress here is that you may no be able to

achieve maximum performance from your watercraft simply by

changing jets in your Super BN. A mismatch of engines components

and or pointing may create a carburetion nightmare. The best advise

is to use quality parts and service from reputable dealers.

To achieve an accurate calibration with a Super BN you should

adjust the tune able circuit in the following order:

1. LOW SPEED ADJUSTER - To adjust a smooth idle

2. POP-OFF PRESSURE - Just off idle to 1/4 throttle in conjunction with

the low speed jet.

3. LOW SPEED JET - Just off idle to 1/3 throttle

4. HIGH SPEED JET - 1/3 to 3/4 throttle.

5. HIGH SPEED ADJUSTER - 3/4 to wide open throttle

The reason for adjusting the circuits in this order is because

several circuits contribute to the total fuel delivery of the carb. Changing

the low speed jet for example, affect wide open throttle fuel delivery to

some degree, ref. Fuel Flow Chart.

The exceptions to the rule are the low speed adjuster and the

regulator portion: The low speed adjuster has no effect past 1/3 throttle.

The regulator portion has no tuning effect past 1/4 throttle, although it

continues to control the fuel supply.

Idle Stop Screw

The idle stop screw is used to adjust the idle speed (rpm) by

opening or closing the throttle valve. As a rule of thumb, adjust the idle

speed to approximately 1100 rpm.

Low Speed Adjuster

The low speed adjuster us used in conjunction with the idle

strop screw to adjust and maintain idle speed and smoothness. Experi-

ment tuning the low speed adjuster in and out in small increments until

a smooth idle is obtained. As the idle stop screw is tuned in our out to

raise or lower idle speed the low speed mixture is also affected.

For clariication, if the idle stop screw is turned out to lower idle

speed, this action increases manifold pressure slightly and richen the

low speed mixture so that a mixture adjustments should be required.

The low speed adjuster is very sensitive and adjustments should be

made in small increments only.


Remember, the low speed adjuster is only for adjust-

ing the idle mixture. If you use the adjuster to help get rid of a low speed

hesitation, you will probably ind that your engine will load up in no wake

zones, or after extended idling.

Pop-off pressure and low speed jet

How do you pop-off pressure and the low

speed jet work together?

These two circuits overlap, although the low speed jet con


tinues past 1/4 throttle where pop-off pressure has little to no effect. In

general, if your pop-off pressure is slightly too high, you can compen


sate by increasing the size o f the low speed jet. The opposite is also

true; if the low speed jet is slightly small, you can compensate with less

pop-off pressure. Once you get to the point where you think each is

adjusted correctly. It’s best to try varying the two to make certain you

have the best combination. For example: If you have pop-off pressure of

30 psi and a 67.5 low speed jet, you should also try a pop-off say 35psi,

and a 70 low speedjet.

To verify that you have the correct combination there are

two things to test:

1. Throttle response should be crisp, with no hesitation.

2. Ride the boat at a constant 1/4 throttle opening for about

1 minute and then quickly open the throttle fully, there should be no

hesitation and the engine should not show signs of being loaded up. If

it hesitates, it’s lean; if it’s loaded up, it’s rich. The irst test is to check

pop-off pressure, the second test is for checking the correctness of

the low speed jet size.

Take the time to ride the boat slowly and thoroughly test your

jetting changes. After a jet change, it takes the engine a few minutes

of use to completely respond to the change.

When does it become necessary

to adjust pop-off?

When personal watercraft come from the factory they have

fairly high-pop due to the fact that they also have somewhat restric-

tive 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 the fuel delivery to the


As you modify or change your watercraft’s lame arrestor to

a less restrictive type you will most likely start to experience a lean

hesitation caused by a decreased in manifold pressure. This change

will require an adjustment in pop-off pressure to regain crisp throttle

response. Because most after market lame arrestors are less restric


tive than stock, you will need to decrease pop-off to compensate.

The Super BN carbs that come from Mikuni American are

already set up for performance applications, and come with pop-off

settings lower than the carbs that come as original equipment.

Pop-off pressure, (the regulator portion of the Super BN) is a

tune able components of the Super BN and works in conjunction with

the low speed jet for good initial throttle response. The components

that make up the regulator portion of the Super BN are:

1. Needle Valve, available in 4 sizes, 1.5, 2.0, 2.3, and 2.5

2. Arm Spring, available in 4 sizes, 115 gr., 95gr., 80 gr., and 65 gr.

3. Arm

4. Regulator Diaphragm

The arm has limited range of adjustment; from the arm

being level wit the adjacent carb surface to being bent upwards no

more than .040” (1 mm) above that surface. If the arm is bent

upwards too much, it can cause the needle valve to be held open

when the diaphragm and cover are installed. If the arm is bent down,

its movement becomes limited and may not be enough to allow the

needle valve to open fully.