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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:

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.

The low speed adjuster is used in conjunction with the idle stop screw to adjust and maintain idle speed and smoothness. electricity diagram flow 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.

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:

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. la gas prices 2016 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 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:

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.

application. Many tuners recommend using 2.3 or 2.5 needle valve in all cases. j gastrointest surg 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.

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. world j gastrointestinal oncol impact factor In either case, make the appropriate jet change and do the complete test again.

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.

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. electricity and magnetism review 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.

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. tropico 5 power plant 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.

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.