Super BN Owner`s Manual - Mikuni American Corporation

Super BN Owner`s Manual - Mikuni American Corporation
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‘performance applications is the
“The se D)
Mikuni. The Su er |
latest i series of wateroraft cerburétors from |
1 ). The fuel pump and jet secti lons are separate for easier mai intenance.
2). Ithas an integral fuel ilter. АЛ TT
3). The fuel pump capacity is s increased by almost 20%.
4). Air flow is increased by 9%. a o Le ee TS oo
>» It is easily tuneable with rep aceable jets (in a wi ide range of Si izes)
“and an array of other tuni ing components. em
With the introduction of the Super BN Series of carburetors,
‘American has made a renewed commitment to the watercraft industry
‘researching carburetor tuni ing ap lications and disseminati ing this
" information to the customer. In the past, jetting informati ion was availab e :
“through many different sources, but unfortunate у, with as many « di fferent
recommendations as there were sources. With this manual, M ikuni
‚ Атепсап | is able to provide a more. rel ¡ab! e and consi istent source of
- approved technic cal ‘information.
Ex cis ise care when he ng ng «
or
any time on your + watércra ina
well ‘venti ated area. ; Do ne t engage the starter whi
lisconnected. edi ate y Le ean up any fuel “spill that
durin j carb install tion. 7 |
E
e fuel lines are
ht occur
d. Te К revie or ng the "harto and dear, of mer you dont
eel I that you can oroperl tion, take your watercraft
о а pete ac na I Proper nstal ation and adjustment wi “SAVE
In most cases four new Super BN Carburetor wil | be installed in
- conjuncti on with an aftermarket mani ifo d and flame arrestor/air fi ter
Sy . We recommend using this manual, together with the ap ¡cable
‘manual for your ‘watercraft, and the instructions from the
aftermarket mani ifold manufacturer to hel you with the instal ton and
C: Super BN carbs were designed to, and work best, in the down draft
position. Many tuners 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 applications:
Piston port and reed valve twin cylinder engines
1). Up to 80 hp SBN 44
2). Up to 90 hp SBN 46
3). 90 hp 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 SBN 44/46’s
E: ALL APPLICATIONS
1). Use a new gasket between the carb(s) and the intake manifold.
2). Use flat 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
fittings. Secure all hose connections with cable ties or hose clamps.
4). If the primer fitting is not being used, plug or cap the fitting.
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 prepared 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 duals; one of the
carbs will need to be modified slightly prior to use. The modification
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 inch (0.125”) 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 for the desired idle speed.
[RS A -
CUT TO
CORRECT
LENGTH
THRUST SPACER
SEAL BOSS
CONIPONENTS
CHECK VALVES
PUMP DI APHRAGM
gusts —
PULSE eo № FILTER
ix RESTRICTER
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FUEL RETURN
FUEL INLET
THROTTLE VALVE |
_ INNER VENTURI
BYPASS \ \
Low speep HOLES
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LOW SPEED ADJUSTER TAR
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HIGH SPEED JET (N102.221)
VENTTO [IR — | =
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ATMOSPHERE __ ) z
— =" LOW SPEED JET (N100.606)
NEEDLE REGULATOR DIAPHRAGM
ARM SPRING ARM VALVE
ASSEMBLY
6 7
UTAVVAY
DRAWING
Use the cutaway drawing (pages 6&7) to learn about the inner
workings of the Super BN carburetor. Starting with the fuel pump, fuel
enters through the fuel inlet, being drawn in by the engine’s pulse
acting on the pump diaphragm. The fuel passes through a series of
check valves and then the fuel required by the engine passes through a
fuel filter. Excess fuel is diverted through a restricter and then back to
the fuel tank via the fuel return. The purpose of the fuel restricter is to
act as a fuel pressure regulator for the fuel pump.
After the fuel passes through the filter it goes directly to the needle
valve assembly and then into the fuel chamber. Fuel flow into the fuel
chamber is controlled by the regulator diaphragm, arm spring, arm,
and the vent to atmosphere. These components, along with the needle
valve, make up the regulator portion of the Super BN. As you will learn,
the Super BN is part carburetor and part fuel regulator. The regulator
portion controls fuel flow at lower speeds, idle to 1/4 throttle, and the
carburetor portion controls fuel flow from about 1/4 throttle to full throttle.
At idle, fuel is drawn through the low speed jet to the low speed
outlet, via the low speed adjuster, and to the bypass holes. The low
speed adjuster controls fuel flow for a smooth idle and initial throttle
response. As the throttle valve is opened from the idle position, the
bypass holes are increasingly exposed to the air flow. Their function is to
help the carburetor “transition” from low speed to high speed operation.
The size of the low speed jet directly affects fuel flow through the bypass
holes for low speed performance. The other function affecting low speed
performance is the regulator portion of the BN. The regulator portion can
be turned by changing the arm spring tension or the size of the needle
valve, or both. More of that later.
As the Super BN transitions to its high speed fuel circuit, at about
3/8 throttle, fuel is drawn through the check valve, and initially through
the high speed jet. The function of the check valve is to prevent the low
speed circuit from sucking in air through the high speed circuit at throttle
openings of less than three eighths. Fuel from the high speed jet passes
through the inner venturi then into the engine. The high speed
adjuster controls the maximum amount of fuel flow for full throttle
performance from about 3/4 throttle opening.
For a closer look at the contribution of each tuning circuit refer to the
Fuel Flow Chart towards the back of this manual.
8
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COMNIPONENTS
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No.Part No. Description
MK-BN38/44 SPR
REBUILD KIT FOR SUPER BN38,
BN44 and BN46
(Kit Includes Items 1-16 to Rebuild One Carb)
“Item 18. Pump Cover
Available in Colors:
Super BN 44/46 Super BN 38
| BN38/547 Gasket, Pump (2 999-642-002-1A-Black 77 € (19) Also used In late
la. 616-72001* O-ring -1B-Silver style Super BN 44
2. BN34/97 Diaphragm, Pump (Clear)
3. В№34/124 Check Valve (2) (Clear)
4. BN34/125 Grommet, Check Valve (2)
5. В№34/100** Diaphragm, Damper
Sa. BN34/129* Diaphragm, Damper
6. В№34/101** Gasket, Damper
6a. €616-92007* O-Ring
7. BN44/146 Filter
8. B30/305 O-Ring, Hi Speed
9. MD12/16 O-Ring, Low Speed
10. KV/10 O-Ring, Needie Valve
11. BN34/123 Gasket, Valve Body
12. BN34/107 Check Valve (Clear)
13. 730-03027 Spring, Arm (115 grams)
14. VM30/97 Pin, Arm
15. BN38/534 Arm (Stainless)
16. BN44/145 Diaphragm Assembly
(ltems 17-43 for Reference Only Not Included in Kit)
17. -BN34/120-30 Screw with 2 Washers,
Stainless-30mm (4pcs.)
“18. BN34/95B Cover, Pump
19. BN34/98** Body Assembly, Pump
19a. BN34/128* Body Assembly, Pump
20. BNa44/109* Stop Screw, Idle
20a. BN34/82* Stop Screw, Idle
21. BN38/39 Washer (Aluminum)
22. BN38/341*" Spring, Idle
22a. BN38/344* Spring, Idle
23. 6826/03” Cap
23a. BN44/118* Cap with Tab
24. 604-25017 Adjuster, Hi Speed, 38/44
24a. BN 44/167 Adjuster, Hi Speed w/ Cross Bar, 38/44
25. BN38/344 Spring, Hi Speed
26. BN38/39 Washer, Hi Speed (Aluminum)
el. BN38/172
28. BN38/343
29. 604-20008”
Washer, Low Speed (Aluminum)
Spring, Low Speed
Adjuster, Low Speed, 44
29a. BN38/477" Adjuster, Low Speed (with Cross Bar),38
29b. BN44/166 Adjuster, Low Speed (with Cross Bar),44
30. BN44/188 Cap
31. 7860-35015-size Needle Valve Assembly (Viton)
32. VM15/285 spring Clip, N.V.
33. BN34/117-GG
34. CW2-0307-G
Plate, N.V. Retainer
Screw with Washer
35. N100.606-size
36. N102.221-size
Jet, Low Speed
Jet, High Speed
37. - BV15/121-05-GG Screw Q ; В
38. BN34/108-GG Plate, Valve
39. BN44/165 Valve Body Assembly a
40. C5-0414-G Screw (2pcs)
41. C6-0308-G Screw
42. В№44/147
43. MCWS-0512
Cover (Stainless)
Screw with Washer (4) (Stainless)
Note: *BN38 & later BN44
"*BN44 only
6 Super BN Throttle Valve
A and Shaft Kits
MK-BN/038 TV for 38mm carbs
MK-BN/044 TV for 44mm carbs
MK-BN/046 TV for 46mm carbs
Kit includes: throttle shaft,
throttle valve, screws, seals,
bushings, heavy duty e-ring, and
instructions
PRINCIPLES OF —
CARBURETION
FUEL REGULATOR PORTION
Because the Super BN is a watercraft carburetor, it cannot vent its
fuel chamber to the atmosphere like a carb with a float system; this
would cause fuel leakage into the hull in the case of a roll-over. The
Super BN’s fuel chamber is contained by the regulator diaphragm, with
fuel on one side and atmospheric pressure on the other.
As the fuel in the chamber is drawn in by engine suction, the
diaphragm moves in response to the depletion of fuel and the effect of
atmospheric pressure pushing against it. The diaphragm contacts and
moves the needle valve arm allowing fresh fuel to enter the fuel
chamber. As fuel enters the fuel chamber it is being pushed in under
pressure from the fuel pump with enough force to push the diaphragm
back against the atmospheric pressure. When the fuel chamber is once
again filled, the diaphragm relieves the pressure on the arm and the
needle valve closes. In this way, the diaphragm mechanism precisely
regulates the amount of fuel in the fuel chamber.
Once again, the regulator portion of the Super BN primarily affects
tuning at throttle openings of 1/4 or less in response to the engines high
manifold pressure. The regulator portion of the BN carbs is a tuneable
component and can be adjusted to most any engine configuration. The
terms most commonly used in this adjustment is “Pop-Off” pressure.
Pop-off is a relative term, it refers to the pressurization of the needle
valve with compressed air, through the fuel inlet, and noting the
pressure necessary to open the needle valve or “pop” it off its’ seat. The
higher the pressure that is required indicates a higher pop-off.
CARBURETOR PORTION
The Super BN starts performing like a conventional carbs as the
throttle is opened through and past 1/4 in that is uses replaceable jets
and adjuster screw to make changes in calibration. At or beyond 1/4
throttle opening, the engine replaces manifold pressures with air
velocity.
What is the difference between manifold pressure and air velocity?
Manifold pressure is the suction generated by the engine against a very
restrictive mechanism within the carb: The closed or nearly closed
throttle plate. As the throttle is opened, the carb transitions from being
controlled by the regulator portion to responding as a normal carb does.
High air velocity, created by the engine as it revs up, appears as a
strong suction (or low pressure) within the carb body. Fuel flow through
the carb is caused by the differential in pressures between the positive
atmospheric pressure on the diaphragm and the suction within the carb
body. Because air velocity creates such a strong suction, pop-off
pressure has virtually no affect on calibration past about 1/4 throttle.
12
y UNING
Accurately tuning a Super BN, for any application, requires a basic
understanding of its functions 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 not be able to achieve
maximum performance from your watercraft simply by changing jets in
your Super BN. A mismatch of engine components and or porting 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 tuneable circuits 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 to1/3 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, affects 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. Refer to your watercraft owners manual for
the correct idle speed. As a rule of thumb, adjust the idle speed to
approximately 1100 rpm.
LOW SPEED ADJUSTER
The low speed adjuster is used in conjunction with the idle stop
screw to adjust and maintain idle speed and smoothness. Experiment
turning the low speed adjuster in and out in small increments until a
smooth idle is obtained. As the idle stop screw is turned in or out to raise
or lower idle speed the low speed mixture is also affected.
For clarification, if the idle stop screw is turned out to lower idle
speed, this action increases manifold pressure slightly and richens the
13
TUNING (cornt.)
low speed mixture so that a mixture adjustment may be required. The
low speed adjuster is very sensitive and adjustments should be made in
small increments only.
NOTE: Remember, the low speed adjuster is only for adjusting the
idle mixture. If you use the adjuster to help get rid of a low speed
hesitation, you will probably find that your engine will load up in no wake
zones, or after extended idling.
POP-OF PRESSURE AND LOW SPEED JET
HOW DO POP-OFF PRESSURE AND THE LOW SPEED JET WORK
TOGETHER?
These two circuits overlap, although the low speed jet continues 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 compensate by
increasing the size of the low speed jet. The opposite is also true; if the
low speed jet is slightly too 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 of say 35 psi. and
a 70 low speed jet.
To verity 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 first 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-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 the fuel delivery to the engine.
As you modify or change your watercraft's flame arrestor to a less
restrictive type you will most likely start to experience a lean hesitation
caused by a decrease in manifold pressure. This change will require an
adjustment in pop-off pressure to regain crisp throttle response. Because
most aftermarket flame arrestors are less restrictive than stock, you will
need to decrease pop-off to compensate.
14
TUNING (corr. )
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 tuneable
component 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., 95 gr., 80 gr. and 65 gr.
3. Arm
4. Regulator Diaphragm
The arm has a limited range of adjustment; from the arm being level
with the adjacent carb surface to being bent upwards no more than .040”
(1mm) 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.
ADJUSTING POP-OFF PRESSURE
Pop-off pressure is adjusted by replacing the arm spring with one of
a different gram rating. Sometimes, in order to achieve the desired pop-off
pressure, it is also necessary to change the needie valve size; keep in
mind that it's always best to use the smallest needie valve size to obtain
the correct pop-off pressure.
MEASURING POP-OFF PRESSURE
You can measure pop-off pressure with a “Pop-off” pump, available
from Mikuni through your dealer, or you can get a relatively good
indication of pop-off pressure by simply using the needle valve and arm
spring guide in this manual. Pop-off pumps are available in three
ranges: 0-30psi., 0-60psi. and 0-100psi.. For most performance
applications the 0-30psi. pump is appropriate.
Pop-off pressures, in psi., from needle valve size and arm spring
combinations: Arm color &
Dull Shiny
Gola Silver Black Silver
115 grams 95 grams | 80 grams | 65 grams
15
TUNING (cont. )
CHECKING POP-OFF WITH A POP-OFF PUMP
1). Attach the pump to the fuel inlet nipple.
2). Cover, or in some way plug the fuel return nipple.
3). Remove the regulator diaphragm to observe the needle valve.
4). During testing, it is important to obtain consistent readings. To
accomplish this, it is necessary to keep the needle valve wet. Use
WD-40 or something similar to wet the needle valve.
NOTE: Don’t use gasoline because of the fire hazard. Protect your
eyes from the spray when the needle pops open.
5). Pressurize the carb with the pump until the needle valve pops open,
being careful to note the indicated pressure. Test the valve 3 times
to assure an accurate reading.
An indication that your pop-off needs to be adjusted is a lean hesitation
when you open the throttle from idle; in the extreme, the engine may even
die. It is much easier to detect a lean pop-off than it is a rich one, so it is
wise to adjust your pop-off until you get it too lean and then back up until the
lean hesitation disappears.
NOTE: It is recommended that you do not use too large a needle valve
for your application. Many tuners recommend using a 2.3 or 2.5 needle
valve in all cases. Actually, we recommended using the smallest needle
valve that gives you the correct pop-off pressure for your engine. A 1.5
needle valve can flow the maximum amount of fuel that the Super BN can
pump, so the only reason to use a large needle valve is to obtain the correct
needle valve and arm spring combination (pop-off) for your watercraft.
HIGH SPEED JET / THROTTLE POSITION AND JETTING
The high speed jet begins contributing fuel at about 3/8 throttle,
overlapping the low speed jet. The high speed jet is the primary tuning
component from 1/2 to 3/4 throttle. As you have probably noticed, tuning
circuit operations are denoted in fractions of throttle openings.. the reason
for this is simple: Carb jetting does not relate to engine rpm or the boat's
speed, it only recognizes how far the throttle has been opened; each circuit
of the carb responds in turn. This is why it's very important, when trying to
diagnose a carb problem, that you identify at which throttle opening the
problem occurs, in order to adjust the appropriate circuit.
The procedure for testing for the correct high speed jet size is the same
as for the low speed, except that you should now hold the throttle at a
constant 1/2 open for one minute, then quickly open the throttle fully to
check engine response. If the engine hesitates, the carb is lean. If the
engine takes a second or two to clear out and then accelerate, the carb is
too rich. In either case, make the appropriate jet change and do the
complete test again.
16
TUNING (Cont)
HIGH SPEED ADJUSTER
The high speed adjuster is the last circuit to adjust. It primarily
controls fuel delivery from 3/4 throttle to wide open throttle. Turning the
screw clockwise reduces fuel flow, counter clockwise increases fuel flow.
The maximum fuel flow is achieved at three turns out from closed.
To test the high speed adjuster it is recommended that you start with
a fresh set of spark plugs, to get quicker plug readings. Unless you have
an exhaust gas temperature gauge, you will have to rely on plug
readings.
You will need to be in an area where you can hold the throttle wide
open for several minutes then chop the throttle and stop the engine just
prior to removing the plugs to read them. Ideally, you're looking for a
nice brown color on the electrode.
Another indicator of proper adjustment is a maximum rpm reading on
a tachometer. If the carb is lean or rich, it won't pull as high an rpm
reading as when it's right on.
VER
FLOW CHART
100%
High Speed
Adjuster
66% A
% of Total High Speed Jet
Fuel Flow A
33%
Low Speed Jet
Pop-off Pressure
1% E. Low Speed Adjuster
idle 1/4 3/8 1/2 3/4 Full
Throttle Position
17
TUNING CONIFPONENTS FOr
THE SUPER BN CARBURETOR
DESCRIPTION
REBUILD KIT
(Consists of all necessary
gaskets and diaphragms)
For 38, 44 and 46.
NEEDLE VALVE SIZES AVAILABLE
Standard replacement
(1.5, 2.0, 2.3 and 2.5)
For use with high alcohol
content gasoline
(1.5, 2.0, 2.3 and 2.5)
ARM SPRINGS
115 grams Gold
95 grams Dull Silver
80 grams Black
65 grams Shiny Silver
LOW SPEED JETS
Sizes 50 thru 150 in increments of 2.5.
HIGH SPEED JETS:
Sizes 50 thru 200 in increments of 2.5
PART NUMBER
MK-BN38/44 SPR
786-35015-(size)
786-35018-(size)
730-03027
730-03027-T
730-03033
730-03030
N100.606-(size)
N102.221-(size)
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19
IPE BN
ACCESSORIES
MK-BN/001 Coupling Kit for dual BN38, BN44, BN46 carburetors.
MK-BN PMP “Pop-OfP” Pump with 30# gauge.
MK-BN PMP-60 “Pop-Off” Pump with 60# gauge.
MK-BN PMP-100 “Pop-Off’ Pump with 100# gauge.
MK-BN38 NV Needle valve replacement kit for Super BN38 contains
2.0 needle valve and 115 gr. spring w/approx. 32 Ibs.
pop-off and 80 gr. spring for 21 Ibs. pop-off.
Instructions included.
MK-BN44 NV 1 Needle valve replacement kit for Super BN44 and
BN46. Contains 2.3 needle valve and 65 gr. spring for
15 Ibs. pop-off.
MK-BN44 NV 2 Needle valve replacement kit for High Performance
Super BN44 and BN46, contains 2.5 needle valve and
80 gr. spring for 15 Ibs. pop-off.
MK-BN38/44 SPR Carb rebuild kit: Includes all gaskets and diaphragms
to rebuild carb; does not include needle valve. For all
Super BN 38,44, and 46
MK-BN/002 Carb wrench - 12 mm (for easy removal/ installation)
MK-BN/003 Carb wrench - 13 mm (for easy removal/ installation)
MK-BN/O38 Throttle valve and shaft kit, SBN38.
MK-BN/044 Throttle valve and shaft kit, SBN44.
MK-BN/046 Throttle valve and shaft kit, SBN46.
HIGH PERFORMANCE CARB KITS
BN44-650-K Kawasaki 650 SX/TS, SBN44.
BN46-650-K Kawasaki 650 SX/TS, SBN46.
BN44-750-K Kawasaki 750 SX/SS, SBN44.
BN46-750-K Kawasaki 750 SX/SS, SBN46.
20
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22
FPERFOPRNIANCE TIPS
THE “LEFT TURN SYNDROME”
You will find in all instances that your watercraft will turn more easily
to the right than to the left. The reasons are basically simple. First,
engine torque constantly places pressure on the hull to turn right.
If your engine’s performance is marginal, you can notice a dramatic
fall-off in power in a hard turn. This power fall-off can’t always be blamed
on the engine, Being over-propped can also cause the engine to slow
enough to fall off its’ power peak. An engine with a peaky power curve is
especially susceptible to a very dramatic power loss in a hard left turn.
Most recently, with the increase of Sportabout racing, there has
been a marked improvement in hull design with a dramatic increase in
“G” forces encountered while turning: over 2.5 G's. In some instances,
such a hard turn can cause a momentary loss of power due to fuel
starvation in the carbs. Jetting changes cannot correct this situation, the
best solution is to rotate the mounting of the carbs 90, so that their
throttle shafts are perpendicular to the crankshaft axis rather than
parallel. To date, this solution to the problem has been 100% successful.
FUEL DRIPPING FROM THE INNER VENTURI AT IDLE
This situation occurs periodically and is easy to cure. What causes
this problem is a combination of two things. First, low pop-off pressure
(due to installation of a 2.5 needle valve with a light spring pressure)
together with an engine that has a substantial vibration at idle
The engine vibration causes the needle valve to leak, which causes
the engine to run even rougher. You can view this occurrence by
carefully looking into the throat of the carb at idle, you will be able to see
the fuel dripping from the inner venturi. In this same way you can also
check to see that the problem is corrected. The cure for the problem is to
increase pop-off pressure until the dripping stops.
ENGINE HESITATION WHEN ACCELERATING AFTER A HIGH
SPEED DECELERATION
You may find it desirable to increase the number of anti-siphon
valves (part no. BN34/107), If you ride very fast and find that you have a
noticeable stumble when re-opening the throttle after a long, high speed
deceleration. This is caused by excess fuel in the carb.
The engine revs fairly high while decelerating, but it uses very little
fuel. The fuel pump still pulses hard, but there is no demand for the fuel.
A small amount of fuel will overfill the fuel chamber, leak through the
high speed circuit and get deposited on top of the closed throttle valve.
This fuel causes a momentary rich condition when the throttle is re-
opened.
The solution is to use one or two additional anti-siphon valves.
Never use more than two extra, and recheck your calibration after
installing any extra valves; in some cases extra valves can adversely
affect throttle response.
23
a A
R35) IKUNI
RACING CARBURETORS
Mikuni American Corporation, 8910 Mikuni Avenue, Northridge, CA 91324
FAX (818) 993-7388
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