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Articles / Runabout / Carburetor Fine Tuning Guide 1998 by Group K

Carburetor Fine Tuning Guide 1998 by Group K

04 November 2019
Article Carburetor Fine Tuning Guide 1998 by Group K

The technicians of Group K intend this document will provide the most current possible fine tuning information for the Mikuni "Super BN" and "I" series, as well as the Keihin CDK II series carbs as they are applied to personal watercraft. This document is an update of our first carb tuning document. We strongly recommend the reading of that document before reading this one.

Happy Reading, The Technicians of Group K

The Goal - "Seamless" Metering

Everyone wants to have a machine with perfectly tuned carburetion, but not everyone knows what that means. Technicians describe perfect carburetion as being "seamless" throughout all throttle settings. That means you can hold the throttle at any setting without experiencing any sputtering (from being rich) or any hesitations (from being lean). Furthermore you should be able to move swiftly from any throttle speed to another, without experiencing any "stumbling" or "surging". 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. Secondly, as engines become more "high breed" (i.e. produce more horsepower per cc) they become much less tolerant of "less than ideal" calibration settings. If you modify your pwc for higher performance, you increase this intolerance for settings that are not "close". Along that same line, high output race machines have a very narrow tolerance for settings that are not "right on the mark".

While you may never achieve absolutely perfect calibration of your carb(s), you can still make big strides toward better reliability and "predictable" operation of your high performance pwc by performing some basic fine-tuning. If you can't get "seamless" metering…"predictable" metering is the next best thing. The purpose of this document is to help you get as close as possible to that point. We will also attempt to dispel many of the myths and "old wives tales" that makes carb tuning appear to be some kind of black magic. 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) have your work cut out for you. In particular, if you are applying a carburetor that does not employ the use of a "bombsight" type fuel atomizer in the carb throat (Buckshot, RedTop, BlackJack, etc.), it's unlikely that you will ever attain seamless overall carburetion. The design of these "non bomb-sight" carbs inherently flows very well at full throttle, however they meter fuel very erratically at low and partial throttle settings. 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. The result of choosing oversized, non bomb-sight carbs is that you may encounter on going difficulties achieving "predictable" metering, especially at low speeds. 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. The carb manufactures have numerous internal "blueprint" variations of each particular carb body. All these variations are designated at the factory as different "blueprint numbers". Over 2 dozen different "blueprint number" variations of the Mikuni 44mm Super BN have been brought into the USA by different oem and aftermarket suppliers. 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.

Along this same line, do not assume that the "jetting" in the large carbs of a new model will work the same in the piece-meal set you are trying to tune in. Each new boat has it's own exclusive internal circuitry that will cause it to meter differently (and require different jetting) from any other "blueprint number" carbs.

Tools of Tuning - For many years, the standard way to confirm good carburetion was via "reading" spark plugs. 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. Because of all the solvents present in the additives of today's new reformulated fuels, a clear "reading" takes much longer (in operating minutes) to get. 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. As for aftermarket tachometers, the two most popular types are the "Tiny-Tach" (about $50), and the PET2000/2500 (about $180). 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.

Before Testing Begins - You may want to review our other carb tuning document for "before tuning" information. 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. The Mikuni filter is a black plastic "bucket" that is fitted underneath the fuel pump side of the carb. 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. However these rubber tips are not "forever", and they often develop slight leaks when exposed to excessive high frequency vibration. 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. We have dubbed the faulty needles that cause this leakage as "dribblers" because of the fuel that visibly dribbles out of the center atomizer 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 … it's a waste of time. 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.

Jetting With ECWI - Electronically Controlled Water Injection (ECWI) systems are becoming more and more popular in high performance pwc exhaust systems. These aftermarket systems offer huge increases in power between 3500-6500 rpm by injecting additional water into the interior of the headpipe during those rpms. While these systems are very effective in improving low-end power, they can also "very effectively" mask a low range metering difficulty. 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. However for modern pwcs to experience "lean mixture" seizures at full throttle, the mixture has to be "very" lean. The digital tachometer can easily show a rapidly decreasing rpm trend that tells the operator that full throttle seizure may be eminent. However, of equal occurrence on modern pwcs, is 7/8-throttle seizure. This "7/8-seizure" takes place on arrangements that rely on large high speed screw openings (2 turns or greater) to operate. 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).

Mid Range "Torque Peak" Seizure - This type of seizure is becoming much more common on modern pwcs. 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. The rpm where ignition timing reaches maximum advance is usually the "torque peak" of the power band. 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 ... and seizure. At Group K we recommend against aftermarket ignition components that have more "advanced" timing curves, because this added advance compounds this same "torque peak" seizure problem. 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. The pumps on the 44 carbs can meet the demands of the larger displacement cylinders, but the flow capability of the fuel petcock (on/off/reserve) valve, and the stock 1100 fuel pick-up tube cannot pass enough fuel to serve this modified motor.

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). To assure that you are not creating a fuel delivery problem, seek the advice of your engine builder and/or carb supplier.

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. We have no interest in describing the differences, we will only intend to draft the best current tuning information that we can, based on the information we have to date…that's just the way R&D work goes sometimes.

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.

The term "pop off pressure" refers to the amount of fuel pressure needed to push the float needle valve away from it's sealing seat. 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. The pressure reached on the gauge when the needle gives way is called the pop off pressure. "Adjusting" the pop off pressure is discussed below. 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. There is no specific "ideal" pop-off pressure for all engines. But when the pop-off is far off of "ideal spec" for a particular engine format, it can cause the other circuits to not come into adjustment or not function properly.

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.

Low Speed Screw - Theoretically, "every" pwc engine (no matter how radically modified) should be able to start with the touch of the button (no throttle required) and then maintain a reasonably steady idle. Even engines with enlarged and modified throats should be able to deliver this type of operation.

The low speed screw should be adjusted to the setting that offers the highest "sustainable" idling rpm. 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 ¼ turn that offers this good idle (about 1300-1400). 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. We say "almost" because the pop-off pressure can affect the setting of the low speed screw that will finally offer the ideal starting and idling that you want.

As an example, an engine with dual 38 carbs may have a pop off pressure of 35 psi, and offer perfect starting and idling with the low speed screws set at 2 ½ turns out. In a perfect world, it's desirable to have the low speed screws end up "in the vicinity of" 1 turn out. The 2 ½ 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. It has been our experience that the pop-off pressure that allows for this "close to 1 turn out setting" will also usually yield the best overall low speed response. If you find your best operation at a setting of less than ½ turn out, the pop-off pressure should be raised slightly. If you find your best operation at a setting above 2 turns out, the pop-off should be lowered slightly.

Needle Valve and Seat - The popular size Mikuni needle valves are 1.5, 2.0, 2.3, and 2.5 (these numbers designate the orifice size in mm. Some stock boats come with other sizes as well: 1.2 in the GP 1200, 1.8 in the 950 Sea Doo. The larger seat diameters will yield lower pop-off pressures. As a rule of thumb, we recommend to use the smallest diameter needle valve/seat that allows the pop-off pressure you are seeking. We recommend this because the smaller diameter seats are less susceptible to vibration leakage/damage, and therefore far less likely to turn into "dribblers". Always use the same size seats in multi-carb arrangements.

Float Arm Spring - These small springs are the preferred way to adjust pop-off pressure because they are inexpensive and easy to change. Lighter weight springs will yield lower pop-off pressures. The standard weights are 65 gm (silver), 80 gm (black), 95 gm (silver but wound the reverse of all the others), and 115 gm (gold). Here too, the springs (along with the pop-off pressure) should not be varied in multi-carb arrangements.

Low Speed Jet - Unlike the low speed jet in many motorcycle carbs, this low speed jet actually has the bulk of it's affect in the mid-range (30-60% throttle). It is common to make significant increases in low speed jet size without having to re-adjust the low speed screw at all (to compensate). The same cannot be said for the pop-off. Changes in the low speed jet will overlap into the pop-off circuit, thus requiring a slight adjustment to the pop-off to attain seamless metering. Unfortunately, even small adjustments to the pop-off (at this time) will require a slight re-adjustment of the low speed screws to regain perfect idling and starting.

The low speed jet is the primary range of affect in situations of "torque peak" seizures. That's because, the low speed jet is the primary circuit of feed when you release to a half throttle "cruise". It often happens that an engine will run reasonably well with a slightly lean low speed jet. This is why torque peak seizures have become much more common. That's also why we recommend to step up on pilot jet size until you experience an apparent mid-range richness that the adjustment screws cannot clear up. If your pop-off/low speed screw combination is well set, you will easily feel the over richness of a lightly oversized low speed jet.

Accelerator-Pump Carbs - Many late model pwcs are coming, from the factory, with accelerator-pump carbs. These carbs have an injector nozzle (fed from the float chamber) that delivers a small spray of raw fuel into the carb throat(s) when the throttle is applied quickly. When the throttle is applied slowly, this system injects little or no fuel. The advantages of this system are many. First and foremost, it allows for a big improvement of low range throttle response on engines that have relatively mild compression ratios. Another big advantage is the reduced sensitivity to slight lean conditions throughout the low speed ranges. The low speed adjustment screws on these accelerator-pump carbs can be set at a slightly leaner setting that permits for very clean and smooth "5 mph zone" operation. Settings this lean (on a non accelerator-pump carb) would likely exhibit a bottom end hesitation when the throttle was applied quickly from idle. When adjusting the low speed screw on these accelerator-pump carbs, we recommend to avoid settings that offer even the slightest rich condition during idling rpms.

High Speed Jet / High Speed Adjustment Screw - While the main jet does effect metering up to 100% throttle, it's primary feed range is the 60-90% range. It can easily happen that a high speed jet can be slightly too lean, yet safe 100% throttle operation can be had by putting the high speed screw to an over rich setting (over 2 turns out). The high risk of this situation is a swift piston seizure if the throttle is relaxed to 80% after a long full throttle pass. At 80% throttle, the high speed screw is not very effective, and it's piston-saving fuel supply is greatly diminished. With the engine still running within 90% of peak rpm, yet the fuel supply cut by much more than that, a piston seizure is not far away.

One method the boat manufactures have used to avoid this risk, is to use a high speed jet so large that the high speed screws must be set at 0 turns out (completely closed that is). This arrangement also eliminates the risk of a customer setting his high-speed screws any leaner than stock. While it sounds a bit unusual, the idea of running closed high-speed adjustment screws has some very sound technical merits (besides the protection against accidental lean settings). Many technicians feel that doing all the high speed metering with the high-speed jet (alone) offers for much more accurate metering of fuel in multi-cylinder applications. While we tend to agree with that concept, making high speed fuel adjustments with the internal jet only is a major pain in the rear. We prefer to make a compromise between these concepts by using main jet sizes that allow for high speed adjustment settings of less than one turn out. We have found that this approach virtually eliminates the occurrence of 7/8 throttle seizures.

Setting The High Speed Adjustment Screw - Few carb adjustments get more attention than the high speed screws. As previously mentioned, we are not big fans of spark plug "reading" as a fine tuning procedure. "Reading" plugs is still a great way to monitor the long term operation of your pwc engine. However, given the high solvent additive content of today's pump gas, plug reading is among the most risky and inaccurate ways to fine-tune the high-speed circuit.

The most accurate means of setting the high-speed circuit is by peak rpm numbers, as indicated by a good digital tachometer. This testing "must" be done on (a large area of) smooth or glass water conditions. With the engine well warmed up, bring the boat quickly up to peak rpm and observe the peak rpm numbers. With a perfectly set high-speed screw, the tach numbers will peak out, then fluctuate up and down within a 30-rpm range. If the mixture is lean, the engine will accelerate strongly up to peak rpm, and then begin to descend steadily without ever stabilizing or raising back up. This situation requires some judgement with respect to the rate of rpm decline. If the rpms decline at the rate of 10 rpm every 5 seconds or longer, the lean condition is slight. If the rpms decline at a rate of 10 rpm every 2 seconds or faster, you have a severe lean condition that can result in seizure if the run is sustained. No matter what the rate of descending rpms, after a loss of 50 rpm or more, the risk of seizure becomes very real. The full throttle pass should be stopped, and a richer setting should be tested.

It bears noting that poor octane fuels (for the compression or rpm range you are running) can also cause a constant trend of declining rpm (a result of the build up of excessive heat). If this happens, and you insist on using the insufficient octane fuel, you will have difficulty stabilizing peak rpm. To avoid the non-stop loss of peak rpm, you will need to set the high-speed screws so over-rich that you will not be able to attain the true rpm peak. This is the performance compromise of low octane gasoline's.

Low-Speed Jet / High Speed Jet Combination - While these two jets (the smaller is the low speed, the larger is the high speed) in the internal chamber of the carb body cover different throttle ranges, those ranges are directly next to one another (along with a certain amount of range overlap). While these jets operate in the same chamber, they have very different operational characteristics. The low speed jet has the brunt of it's impact in the mid range, and delivers fuel all the way to 100% throttle. If you increase a low speed jet from a #70 to a #80, that additional increment of 10 is also present at 100% (full) throttle. This means that if you wanted to increase the fuel delivery though the entire middle and upper throttle range (all the way to 100%), it can be done solely with a low speed jet change. This also means that an increase in low speed jet size would require an equal decrease in main jet size if you wanted to retain the same mixture, as before, in the high range.

Uneven Jet Sizes - Before you embark on re-jetting your pwc, you would be wise to disassemble the carbs to determine the exact jetting of each individual carb. Most of the new pwcs are coming from the factory with different size high and low speed jetting for different cylinders. The term for this is "staggered", or un-even, jetting. Our web document "Rear cylinder Piston Seizures" covers the reasoning behind this in detail. As a rule, we recommend to maintain the "stagger" of the factory jetting when increasing high or low speed jets.

About Take-Offs - Perhaps the biggest area of difficulty, with respect to jetting, is accommodating the different kind of take-off styles of recreational riders. Keep in mind that most professional tour racers perform relatively high rpm starts with two powerful holders controlling the machine. For them, seamless low speed metering that can cleanly and instantly accelerate away from idle or 5 mph zones is a total non-issue. As a result few modified racing carbs, and few tuners hold these abilities as a high priority. In the "real world", those are very big priorities.

While seamless carburetion is certainly supposed to result in instant acceleration under any conditions…that cannot always happen. The most difficult scenario is what we call the "Saturday-Nite" take-off. As it infers, the operator wants to position next to his buddy while setting still in the water, then bolt away by instantly grabbing the throttle to 100%. This may have worked out well for 60's musclecars, but it seldom works out well for recreationally modified pwcs. When a pwc accelerates casually away (instead of bolting) from a Saturday-Nite start, the first thing the owner thinks is that he is under powered or his carb adjustments are off. While one of these may be true, there are many other contributing factors as well.

For many of the new larger and heavier pwcs, there is a tremendous amount of wetted hull surface when the engine is at idle. When the hull is on plane, a big percentage of that wetted surface no longer has contact with the water. This additional water contact surface area (at idle) adds a lot of surface drag that significantly slows down the rate of acceleration in those first few moments of the SN start. This same heavy machine, just barely up on plane, will accelerate away with all the authority that the owner is looking for.

An additional factor for many machines is the lack of water pressure to the headpipe during a "SN" take-off. Most high performance pwc owners are aware of ECWI systems (electronically controlled water injection). As previously mentioned, these aftermarket systems offer huge increases in power between 3500-6500 rpm by injecting additional water into the interior of the headpipe during those rpms. This water is injected to supplement the (low-end assisting) water already being supplied to the headpipe by the stock cooling system. However when a rider positions in the water (at idle speeds) next to his buddy, the water pressure being delivered into the headpipe is very low to non-existent. As a result, there is no "low end" enhancing water pressure in the headpipe when the throttle is instantly applied. The result is very weak low range acceleration when the throttle is suddenly snapped full open. Here again, having the machine just barely up on plane assures that there is an adequate supply of water in the headpipe to offer the bolting take-off that the rider was looking for.

Even with all this, there are some situations where a "planing" low speed start can still yield a take-off that is "mediocre". In many of these cases, the problem can easily be resolved by grabbing only partial throttle on the initial take-off, then drawing to full throttle once on plane. This effective tip works so well because, at partial throttle (50-70%), the carb butterfly position allows for strong inlet tract vacuum that picks up fuel very efficiently under heavy loads. The sudden low inlet tract vacuum, caused by an instant opening to 100% throttle, does not pick up fuel from the fuel circuits very efficiently, therefore does not offer very strong initial acceleration.

Modified Throat Carburetors - Many shops, Group K included, offer modifications to the throat diameters of various models. Of these modifications, we categorize them in to two basic groups, "bomb-sight atomizer" and "non bomb-sight atomizer". There is little doubt that Non-bsa designs (BuckShot, RedTop, BlackJack, etc.) have better cfm (cubic feet per minute) flow "at full throttle" than the bsa designs. Unfortunately, all this flow typically comes with very significant compromises and difficulties in seamless low and middle range operation. Furthermore, because the non-bsa designs do a very poor job of directing the partial throttle air directly over the low speed transition circuits, they are notorious for requiring constant adjustment for subtle changes in air conditions. We have yet to see any non-bsa design (of same diameter) that can match the overall performance and user friendliness of the bsa designs. For detail on this subject, see our Sea Doo 785 Updates document for the entry titled "Large Carbs Pt 2".

Of all the aftermarket carb modifications, we consider the latest designs from Novi (in North Carolina) to be the top of the heap. The thing that sets the Novis apart from the others, is the intent to improve overall performance by improving overall metering and mixing. The accent is on the maximum efficient overall operation, not on "maximum cfm no matter how big the compromises are". The Novi design uses their own patent pending bomb-sight (they use the automotive term "booster venturi") design that is intended to dampen the reverse waves from the lower end that cause an uneven delivery of fuel at various rpms. It also makes for increased signal strength at the main jet. The net result of this dampening is much cleaner overall carburetion, as well as a higher peak rpm ability. The higher rpm comes from the reduced tendency for the carb to "go rich" at high rpms. "Going rich" is the term for a carb that is losing it's ability to precisely meter fuel in the very high rpm range (largely due to those strong reverse inlet tract pulses).

Overall, these carbs will be far less vulnerable to changes in air density than the non-bsa carbs. The will also rpm as well as the larger throat non-bsa designs, without taking on the low inlet tract airspeeds that hamper low rpm response. As an added benefit, the Novi design will get considerably better overall fuel consumption (than the non-bsa's) because of the much better atomization at all the partial throttle settings.

By Harry Klemm -


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