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Super BN
Introduction
Introduction to Super BN Carbs
The Super BN Carburetor for high performance application is the latest in a series
of watercraft carburetors from Mikuni. the super BN earns its name because of the
increased performance it delivers over previous Mikuni BN Carbs.
What differentiates the Super BM from previous BNs?
1) The fuel pump and jet sections are separate for easier maintenance.
2) It has an integral fuel ilter.
3) The fuel pump capacity is increased by almost 20%.
4) Air low is increased by 9%.
5) It is easily tunable with replaceable jets (in a wide range of sizes) and an
array of other tuning components.
With the introduction of the Super BN Series of carburetors, Mikuni American has made
a renewed commitment to the watercraft industry by researching carburetor tuning applications
and disseminating this formation to the customer. In the past, jetting information was available
there were sources. With this manual, Mikuni American is able to provide a more reliable and
consistent source of approved technical information.
Carb Installation
CAUTION:
Exercise care when changing carburetion compo-
nents and anytime you work with the fuel system on your water-
craft. work in a well ventilated area. Do not engage the starter
while fuel lines are disconnected. Immediately clean up any fuel
spill that might occur during carb installation.
A:
As when installing any performance product, a degree of
mechanical ability is required. If after reviewing the parts and
instruction you don’t feel that you can properly complete this in
-
stallation and adjustment will save time, money and aggravation.
B:
In mos cases your new Super BN Carburetor will be installed
in
conjunction with an after-market manifold and lame arrestor/air
ilter
system. We recommend using ths manual, together with the
applicable shop manual for your watercraft, and the instructions
from the aftermarket manifold manufacturer to help you with the
installation and tuning of the Super BN Carburetor.
C:
Super BN carbs were designed to, and work best, in the down
draft position. Many turners choose to mount Super BNs in more
of a side draft position. This will work okay, as long as the carbs
are oriented with the pump on top and the regulator portion down,
otherwise the jet portion will constantly drain into the carb making
calibration impossible.
D: How to determine the correct carb size to
use.
The following recommendations are for
performance application:
E: All Applications
1)
Use a new gasket between the carb(s) and the intake manifold.
2)
Use lat washers and self-locking nuts to secure the carb(s).
3)
Be sure that the fuel lines are of the correct diameter for the carb
ittings. Secure all hose connections with cable ties or hose clamps.
4)
If the primer itting is not being used, plug or cap the itting.
5)
When installing the throttle cable, be sure to securely tighten the jam
nuts. Allow free play at the throttle lever. Make sure that the throttle
cable is not pulled open as the handlebars are turned from side to side;
correct the cable routing if necessary.
F: Installing Dual Super BN Carbs
The only Super BN carbs that are factory prepare to be used
as duals are the BN38-34-8075, 8076. Several other models have extended
throttle shafts to make them easier to use as dual; one of the carbs will need
to be modiied slightly prior to use. The modiication consists of removing the
throttle lever, idle stop, spring and sleeve, and then cutting the threaded shaft
end to the desired length. After removing the above mentioned parts, place
both carbs on the manifold and cut enough of the threaded end of the shaft
to allow one eighth inc (0.1257) clearance between the shaft ends: Be sure
to chamfer the sharp edge.
A Mikuni coupling kit, MK-BN/001, is recommended to connect the
two carbs together and synchronize them.
Note: The sleeve that was removed from the carb is actually a thrust spacer
and must be used with the coupler. The sleeve should be shortened to the
correct length and placed between the end of the coupler and the seal boss
(see illustration) to prevent the shaft from moving side to side; this movement
can cause damage to the throttle valve.
G: SYNCHRONIZING DUAL CARBS
1) Place the coupling on the throttle shaft between the carbs and secure
the carbs to the manifold.
2) Back out the idle stop screw until it no longer contacts the throttle
linkage.
3) Position the coupling between the carbs, against the shortened sleeve, and
securely tighten the set
screws on one end only.
4) Use the eraser end of
a pencil or similar blunt
object to push and hold
the throttle valve closed in
the carb without a return
spring. With the throttle
valve held closed, tighten
the remaining screws.
5) Adjust the idle stop
screw ir the desire idle
speed.
Mikuni Super BN46 carburetor
550
1.) Up to 80 hp
SBN 44
2.) Up to 90 hp
SBN 46
3.) 90hp and up
Dual SBN 38, 44, or 46’s
Rotary Valve engines
1.) Limited class
SBN 44/46’s or dual SBN 38’s
2.) Mod class
Dual SBN 44/46’s
Three Cylinder engines
1.) Limited Class
Three SBN 38’s
2.) Mod Class
Three SBN44/46’s
Piston Port and reed valve twin cylinder engines