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.
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.