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A must for any Super BN carb user. Covers specifications, tuning and repair for all Super BN series carbs Please upgrade your browser or activate Google Chrome Frame to improve your experience. You must have JavaScript enabled in your browser to utilize the functionality of this website. Part Number: 01-02-101 This site contains original copyright photographs and content that is the sole and exclusive property of Blowsion Kolors. Any unauthorized use of content found on this site without written approval of the owners will be prosecuted to the full extent of the law. Use of this web site constitutes acceptance of the Blowsion Kolors User Agreement and Privacy Policy. Extra Info This product was added to our catalog on. Notify me of updates to Mikuni Super BN Manual Tell someone you know about this product. Advanced Search. Download and read or print these documents by clicking on the titles below. General Tuning Information Tuning Manual (1.3 MB) When you click on one of the manuals in this list, Adobe Reader will load and proceed to download the chosen manual. This could take some time on slow connections as some of the manuals are rather large. Their size is noted to the right of each manual title. We recommend you down load the HSR tuning books for Harley Davidson. This is perhaps one of the best written book on carb tuning, just ignore the accelerator pump section if using a non pumper carb and remember that the mixture screw is an air screw where as on carbs with the screw located behind the bowl it will be a fuel screw in which case winding out is to make richer not leaner. By following the procedures in the book you will determine if jets need changing and in what direction. Order required jets as your checks tell you. Please positively identify your jet type in the jets section, as all jet sales are final with no refunds. If at all in doubt, see your local tuning shop. Wind back ? the number of counted turns to the centre point.
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Then count the turns from this point to bottoming out, if you take less than 3 and more than 1 complete turns to achieve bottom, the jet is correct and you simply wind back to the centre point. Job done. Remember if you don’t wait 10 seconds for the engine to catch up every half turn at a very slow idle this tuning process will not work. Some engines will actually have two size jets that may fall within the 1-3 spec in which case see which feels better when riding, generally the richer one. The performance changes can be detected at wide open throttle in low gears- we do not recommend the operation of motorcycles over the speed limit. Don’t give up half way through this process thinking you’ve got it right before reaching the limit, otherwise you may will be robbing yourself of a surprising amount of horse power! We don’t do returns on jets so maybe buy one bigger and one smaller to determine which way you need to go before then buying a selection in the correct direction (richer of leaner). Never drill jets and do remember that most aftermarket jets do not flow correctly. Generally if you have the right profile needle for your engine, a difference in performance and response can be detected by raising and lowering the needle. If improvements appear in one direction of adjustment but perfection is not achieved whilst having reached the end clip on the needle, then you will need to install a richer or leaner needle if available or else a corresponding needle jet. Once having made a change its back to the middle clip position and start all over again. Never change main or pilot jets to try and correct a mid-range problem. 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.
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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. The reason for adjusting the circuits in this order is because several circuits contribute to the total fuel delivery of the carb. IDLE STOP SCREW The idle stop screw is used to adjust the idle speed (rpm) by opening or closing the throttle valve.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. 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.
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5 low speed jet, you should also try a pop-off of say 35 psi and a 70 low speed jet. 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. 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.
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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. 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. As you have probably noticed, tuning circuit operations are denoted in fractions of throttle openings.
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. 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. 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. 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.
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. 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. Something went wrong.Get the item you ordered or your money back.User Agreement, Privacy, Cookies and AdChoice Norton Secured - powered by Verisign. 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. The high speed jet is the primary tuning component from.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.If the engine hesitates, the carb is lean. 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. 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.This document is an update of our first carb tuning document. We strongly recommend the reading of that document before reading this one. That means you can hold the throttle at any setting without experiencing any sputtering (from being rich) or any hesitations (from being lean). Starting should always be instant, idling should always be smooth and steady, and acceleration characteristics should always be consistent and predictable. In the real world, all this doesn't happen too often?even on stock boats. The skilled carb specialists that work for the boat manufactures have a tough time meeting all these goals on stock boats for two basic reasons. First, and foremost, there is a wide variation in air density due to variations in altitude, air temperatures, and weather conditions. Setting up one carb that can work perfectly under all these various conditions is impossible. The purpose of this document is to help you get as close as possible to that point. No wand is required.
Carb Choices - If your machine has stock carburetors (along with some other mild modifications), the job of fine tuning will not be too difficult. If you are installing an aftermarket carb of some kind (that has not been pre-set for your engine arrangement).you have your work cut out for you. This erratic low speed metering can be minimized if you choose a throat size that is not much larger than stock. However few buyers of aftermarket carbs seem to make that choice. This is no big deal for pro racers, but it can be a very big deal for just about everyone else. Piece-meal Carb Sets - Lots of enterprising back yard mechanics enjoy constructing, and dialing in, their own carb setups. These setups are often a collection of a carb from this buddy along with another carb from that buddy, and a manifold from yet another pal. The biggest risk involved with this kind of piece-mealing is variations in the carburetors themselves. An example is a customer who used a 44 Mikuni off his Blaster, and another 44 Mikuni off his buddy's 650 Super Jet to make a dual 44 carb kit. While the two 44 carbs looked identical externally, the internal circuitry drilled into the carb bodies (from the factory) was very different. This hidden variation between the two carbs made it impossible to get them to meter the same in this dual carb application. If you intend to piece-meal together a carb kit for your machine, do whatever you can to assure that the carbs your starting out with are as identical as possible. While that procedure wasn't perfect, it was a good as any other procedure. Today, reading plugs is a very vague (and risky) way to confirm carburetion. By then you can have already seized a piston. By far the most accurate and effective tool for carb tuning is a good digital tachometer. The digital tachometers found on Sea Doo and Polaris pwcs have excellent accuracy and update times. We highly recommend that owners depend on them. The Tiny Tach updates the rpm about every 2.5 seconds.
This means that the Tiny tach must see the same sustained rpm for 2.5 seconds to yield an accurate number. While that can be suitable for general recreational use, we consider 2.5 seconds to be too long for effective carb tuning. The PET tachometers update twice a second (like the stock Sea Doo and Polaris tachs). This quick update is an essential feature for safe high speed tuning?particularly on race engines. Here, we will add a couple of additional inspections. In-Carb Fuel Filters - Both Mikuni and Kiehin carbs have internal fuel screen filters that must be clean before tuning. In the Kiehin, the filter screen clips directly onto the bottom of the brass fuel needle and seat. Needle and Seat condition - Virtually all pwcs now come with float needles that are fitted with rubber tips that seal against the brass fuel inlet seat. These rubber tips offer much better sealing than the metal tips used only a few years ago. Revving a pwc engine into a higher rpm range contributes to this kind of vibration. When a float needle becomes damaged, it will usually begin to leak a small stream of excess fuel into the carb throat at idle speeds, and during throttle release from high rpms. This leakage can easily be seen while looking down the throat of the carb at idle speeds. It is impossible to attain seamless or predictable metering with a dribbler in a carb. We have seen countless owners spend all day trying to tune away a low speed rich condition caused by a dribbler?you will never do it. Keep in mind that many heavily modified IJSBA tour machines vibrate so intensely that tuners will change the float needles in between races to avoid the risk of a poor start caused by a dribbler. Don'ts - DO NOT attempt any kind of carb fine tuning in rough water conditions.Smooth water allows you to feel (with great accuracy) when you have made metering better or worse. Smooth water also allows your tachometer to yield the most accurate readings.
DO NOT change any engine parts, exhaust system adjustments, fuel mixes, or octane levels during the course of a test. All these factors can cause significant changes in metering. DO NOT attempt carb fine tuning on a machine that is over propped. The excessive load of an over-pitched prop will make it impossible to feel subtle changes in throttle response and acceleration. This masking becomes so profound that low speed fine-tuning becomes almost impossible to confirm. To avoid this masking, we recommend that you do as much low range jetting as possible with the ECWI system disconnected. This is easily done by disconnecting the wires between the solenoid and the driver. The rest of the ECWI plumbing can remain in place without creating any problems. Once your jetting is completed, re-connecting the wires will restore the full function of the ECWI. Seizures - Perhaps the greatest fear that most folks have, related to carb tuning, is the risk of accidentally inducing piston seizure. This fear is well founded, but the true nature of the seizures is usually misunderstood. The sophisticated ignition curve of modern pwc reduces the likely-hood of piston seizure related to slightly lean high speed mixtures, but seizures that are a direct result of detonation have become much more commonplace. Power Peak Seizures - This is the failure that used to be most common. The digital tachometer can easily show a rapidly decreasing rpm trend that tells the operator that full throttle seizure may be eminent. While this subject is covered later in this document, we would recommend that any machine with high speed screws set beyond 2 turns out be fitted with a richer main jet (5 - 10 numbers richer). The newer boats have ignitions that are very retarded at idle speeds, then reach their advance peak around 6000-6200 rpm. After the peak advance, the timing retards slightly as rpms escalate.
That means that the engine is generating more sheer torque, per revolution, than at any other engine rpm. If your carburetion has an extreme lean condition in the mid-range, you can ride full throttle all day long. However the first time you release the throttle into that lean middle range, the combination of the lean mixture, and the heavy timing advance, can create detonation that will result in a swift piston seizure. The rider seldom suspects a lean mid range, and begins looking for other gremlins (air leaks, etc). It bears noting that a prop with too much pitch can cause exactly the same problem. The higher pitch will cause the rpms to be significantly lower (closer to the torque peak) at full throttle. The lower rpm number means more advanced timing, at full throttle, that can (once again) result in detonation. We seriously question the benefits of any additional ignition advance (over stock) on modern pwcs, particularly in cases where pump gasoline is being used. Fuel Metering vs Fuel Delivery - We think it's very important to distinguish between fuel metering and fuel delivery so that their symptoms are not confused. Fuel delivery is the function of the fuel tank and fuel pump that refers to the adequate supply of fuel (and fuel pressure) to allow for precise metering by the carb(s). Fuel metering refers to the function of the jet circuits in the carburetor accurately delivering the correct amount of fuel for good operation. It often happens that a fuel pump, or fuel system, does not provide enough fuel delivery to allow for correct metering. A perfect example is 1100 Yamahas that have been fitted with big-bore top end kits, and 44mm carbs. Another common example is the 785cc Sea Doo models. The stock fuel pump on these models is rated for 10 gallons per hour of output. This is plenty of fuel for moderately modified machines.
However if you install an exhaust system that increases rpms beyond 7200, the stock pump will be at (or beyond) it's ability to deliver enough fuel for the engine to operate at peak rpm. The result can be intermittent fuel starvation at peak rpm (a very difficult problem to diagnose). Mikuni Pumps - The Mikuni fuel pumps (both remote and carb mounted) are very efficient units. However they can be susceptible to damage by engines whose lower ends have been filled by fluid (fuel or water). A machine that has been sunk will usually have the entire lower end of the motor filled with water. If the start button is pressed on such an engine, the pistons will hydraulically lock against fluid in the lower end. In the Mikuni fuel pumps, there are two small, round siphon diaphragms made of a clear plastic. When the force of a hydraulic lock from the lower end makes it's way up the pulse line to the fuel pump, these two diaphragms take a heavy sudden impact. This impact can buckle (or crease) these diaphragms in the pump. If these diaphragms are damaged, in any way that compromises a perfect seal against the aluminum pump surface, the pump's efficiency can be seriously compromised. In other words, if your lower end has ever been filled with liquid, these diaphragms in the fuel pump must be closely inspected. Air Conditions - The prevailing air density of the moment plays heavily into the performance ability of a high output pwc engine. Heavy air density is offered by low altitudes, low air temperatures, low humidity, and high barometric pressures. Of these, the variable that changes performance (and carb settings) most is air temperature. It often happens that a machine, that got it's last fine tuning session in the heat of late summer, is run for it's first annual outing in the frigid (and very oxygen rich) air of early spring. The relatively lean settings that were ideal for last summer are usually much too lean for safe operation in the oxygen rich air of early spring.
Add to this the low specific gravity of heavily oxygenated winter fuels (which further leans out mixtures) and you have an ideal recipe for a pre-season piston seizure. If you plan to operate a high output pwc in a wide range of air temperatures (or air densities), be wary of the fuel demands that will accompany those weather conditions. The Circuits - Since the writing of our last carb tuning document, much new specification information and test data has caused us to re-examine the range of effect of the various circuits. Some of the following information, as it reads, conflicts slightly with our previous carb tuning document. Pop Off Pressure - The pop off pressure is not an adjustment that exists on the carb, but rather a specification that is a combined function of the needle valve size, and the spring rate of the float arm spring. This specification has a wholesale effect on the fuel metering in the 0 - 40 fuel range. Because the pop off pressure has this far reaching range that overlaps with several other metering ranges, we consider it a fundamental starting point. Pop off pressure is checked with a hand pump that is fitted with an in line gage. The pump is connected to the fuel input fitting of the carb. The return line fitting is then sealed off with one finger while the pump pressurizes the float chamber. If you don't have a pop off pressure gauge, you should get one that has a gauge and pump capable of 30 psi. (Most Mikuni distributors carry them) At the beginning, it's only important to check that the needle holds the pressure back with no leaking up to the point where it pops cleanly away from the seat. Perform the pop off test several times to confirm the actual pop off pressure. Initial pop off pressures on the Mikuni and Kiehin carbs (unless otherwise specified) should be no less than 25 psi and no greater than 35 psi.
We will address pop-off adjustment again later in this document, but the pre-existing effect of the pop-off on other ranges must always be kept in the fore. In short, the pop-off pressure should be changed if a collection of low range circuits cannot accommodate the current pop-off specification. The best indicator of this is the setting of the low speed adjustment screw. To be Continued. This setting is easiest found by viewing a digital tachometer with a warmed up machine running in the water (tied to a trailer is fine). As richer and leaner settings are tried, you will quickly see the trend toward higher idle rpms. In truth, you will see the highest rpm when the low speed screws are slightly too lean. However you will notice on the tachometer that those high rpms will waver up and down, not steady and sustained. You will eventually find a range of about.Within this adjustment range, find the setting that allows the engine to come down to the same steady idle rpm after you snap the throttle open momentarily. An additional test of the perfect setting is to stop the engine (about 30 seconds), then restart without touching the throttle. In this test the engine should come back to it's normal steady rpm number. The low speed adjustment screw carries it's greatest impact in the 0 - 20 throttle opening range. It continues to have a lesser impact up to 40 opening. The low speed screw setting is almost wholly responsible for allowing easy starting and steady idling. The 2 ? turns of our example is actually an excessively rich compensation for a pop-off pressure that is too high. By reducing the pop off pressure, with a lighter pressure float arm spring, the entire 0 - 40 range is slightly richened. This additional richness will allow for a much leaner (closer to 1 turn out) screw setting that will offer perfect starting and idling. If you find your best operation at a setting of less than.