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
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.