Carb Tuning
Accurately tuning a PWC carburetor 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
carb 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 the carb. 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 carb 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 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, affects wide open throttle fuel delivery to some degree 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 our 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 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-OFF 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 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 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 first test is to check pop-off pressure, the second
test is 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.
The Super BN carbs that come from Mikuni America 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 (Note: Some OEM
carbs have 1.2)
2. Arm Spring, available in 4 sizes, 115gr., 95gr., 80gr. 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 needle valve size; keep in mind
that it's always best to use the smallest needle valve size to obtain the
correct pop-off pressure.
MEASURING POP-OFF PRESSURE
AERO CORS-AIR USA
You can measure pop-off pressure with a "pop-off" pump, available from Mikuni
through your dealer.
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 2.3 or 2.5 needle valve in all cases.
Actually, we recommend 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
larger 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 ½ 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 ½ 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.
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 (Factory
Pipe suggests that you only do this for about 30 seconds, longer times with
a lean setting could cause engine damage) 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.
PERFORMANCE 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 falloff in power in a hard
turn. This power falloff 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
Sport and Runabout 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 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 deg, so that their
throttle shafts are perpendicular to the crankshaft axis rather that 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 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 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#
BN34/107), If you ride very fast and find that you have a noticeable stumble
when reopening 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 reopened. 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.
Carburetor info courtesy of Mikuni. For a complete Mikuni Super BN Owners Manual
send $5.00 to: Mikuni
American Corporation / 8910 Mikuni Ave / Northridge, CA 91324.
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