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Articles / Runabout / Carburetor Fine Tuning Guide

Carburetor Fine Tuning Guide

04 November 2019
Article Carburetor Fine Tuning Guide

While there seems to be an abundant number of folks selling high performance carburetors and carb kits, there seems to be a desperate shortage of folks providing "understandable" carburetor tuning information. For the 8 or 9 warmer months out of the year, getting a knowledgeable technician to talk to you on the phone about adjusting "your carb on your boat" is darn near impossible.

For the knowledgeable and experienced (read: very very busy) technician, few things are more frustrating than trying to explain fine tuning procedures, along with the history of carburetors, over the phone. It's even more frustrating if that same technician knows it's a carburetor you bought from somebody else (someone who won't help you tune it).

The following is a guide to help you avert being that unwelcome caller. Good technicians, no matter how busy, are usually glad to help someone who has covered all the basics and just requires detail information. The following guide is an easy to understand outline of "those basics". We hope you find them helpful.


The two generations of carbs - Before 1989, virtually all pwc's utilized the "round pump" Mikuni carbs. These carbs came in 38mm and 44 mm sizes only. These "round pump" carbs performed well, but they were somewhat temperamental because the round diaphragm pumps often had difficulty supplying enough fuel to high output racing engines. In 1990, both Mikuni and Keihin introduced "square pump" style carbs. The fuel pumps on these carbs produce more than double the fuel pressure of the earlier "round pump" designs. Among other new design features, the square pump carbs also have changeable high speed and low speed jets. These changeable internal jets allow for very accurate mixture adjustment on a broad range of engine formats.

External adjustments - Virtually all pwc carbs have a high speed and low speed fuel mixture adjustment screws. The adjustment screw positioned closest to the air intake (top) is always the high speed fuel mixture screw (30%-100% throttle range). The adjustment screw closest to the mounting surface (bottom ) of the carb is always the low speed fuel mixture adjustment (0-35% throttle range). As these screws are turned out, the fuel mixture is made richer. All adjustment settings are noted as "turns out" from the bottomed out position. That is, "1 turn out" means 1 turn from the bottomed closed position.

Power tuning - Many shops offer "power tuning" as a means of adjusting carburetion. The boat is held stationary in a test tank or on a trailer backed into the water so that adjustments can be made while the engine is running under a load. This type of tuning is adequate for getting carburetion close, however it is by no means an effective way to achieve the ideal mid-range or full throttle carb settings. Power tuning does not simulate the added loads of the water drag on the hull surface, the rider's weight, or high speed water being loaded into the front side of the pump. These collective loads make "riding on the water" the only accurate way of evaluating carburetion settings on a high output watercraft.

Reading spark plugs - Determining proper fuel mixture by inspecting the color and condition of the spark plugs can be very helpful in situations where the engine is being operated constantly at full rpm under full load. "Reading plugs" for perfect fuel mixture is very common in high speed auto and motorcycle racing where the engines are nearly always run at full rpm and full load. Closed course pwc racing, however, requires as much "partial throttle" operation as full throttle. Furthermore a pwc racing engine seldom experiences full steady loads because of the rough water conditions. This means that spark plug readings, done on a pwc that is being ridden on a rough water course, has very questionable accuracy.

To get an accurate plug reading on a pwc, a fresh set of spark plugs should be run in the machine for 3-5 minutes at full throttle/full rpm on relatively smooth water. At the end of the full throttle running, the throttle should be chopped and the kill button pushed simultaneously (called a "plug chop"). If the engine is run at partial throttle for even 3 seconds after the full throttle run, the plug reading will be invalid.

After the full throttle running, and the plug chop, a combination flashlight/magnifying glass must be used to view the carbon deposit at the base of the porcelain (down inside the spark plug where the porcelain insulator and outer steel spark plug casing meet. A ring of dark brown at the base of the porcelain denotes ideal fuel mixture, light brown is lean, and a ring of black is over rich. This is the only area of the spark plug that accurately indicates fuel mixture. Furthermore, this reading only indicates full throttle fuel mixture. No part of the spark plug can indicate low speed or mid range fuel mixture. The upper part of the spark plug porcelain (by the electrodes) is often very light or white in color, however this coloring is mostly affected by additives in the gasoline and oil. The coloring of the end of the porcelain in no way indicates appropriate fuel mixtures of any throttle range. The cosmetic appearance of the spark plugs can defiantly help a pwc mechanic to quickly diagnose the symptoms of a major operational problem. But as far as carb fine tuning for personal water crafts is concerned...reading plugs qualifies as a very questionably accurate way to fine tune the carbs. Very few professional PWC engine builders recommend their customers to do carb fine tuning based on plug readings...and even fewer engine builders do it themselves.

The weather - Weather and altitude can defiantly be a factor during fine tuning. The factors that will require you to go leaner are, higher altitude (changes of 1000 ft. or more), higher temperatures (changes of 20' F or more), and higher humidity (changes of 20% or more). Water temperature itself (55-85'F) seems to have very little effect on fuel mixture. It seems that the big changes in weather that come with very warm water, and very cold water are what actually affect the mixture.


Air leaks - The lower end of a two cycle engine must be air tight to about 10 psi. If there are any minute air leaks at a crank seal or a gasket surface, tiny amounts of air will intermittently leak into the lower end and cause a temporary lean condition. As a matter of reality, about 50% of the engines on an average race lineup have an air leak. Most of those leaks are not big enough to cause chronic hard-starting or piston seizures, however they are usually big enough to cause on-going jetting problems.

As the castings of an engine expand and contract with heat, so too can the air leaks change to admit greater and lesser amounts of outside air during operation.

Group K offers an inexpensive pressure test kit that allows you to quickly check for, and locate, any potential air leaks your engine may have. An engine with a small air leak will never carburate consistently. Remember...air leaks never get smaller.

Reeds - If your reed petals are chipped or frayed in a way that does not permit perfect sealing, the low speed and mid range circuits will be very difficult, if not impossible, to set accurately. Damaged reed petals will cause a false low speed rich condition, not to mention hesitations in mid range that you will not be able to carburate out. Installing aftermarket reeds will often require significant changes in cab adjustment.

Carb gaskets - Confirm that these gaskets have a soft drying sealer (like Gaskacinch or Permatex Hi-Tack or 3Bond 1211)on them, and that the carb mounting bolts are torqued.

Confirm full closing and opening - With the flame arrestor(s) off, be sure that the carb butterfly(s) can close completely with the handle pole all the way down and the handlebars in the full left and right positions.

Pressure test fuel system - All pwc utilize a sealed fuel system that has a check valve on the gas tank vent. This check valve (which permits pressure in the gas tank but not out) causes pressure in the gas tank that helps deliver fuel to the carb(s). Any air leak in the fuel system that permits the leaking off of this pressure, will also affect fuel delivery to the carb(s) at low speeds. To test for leaks, follow this procedure. 1) remove the return line from the fitting on the carb. 2) Blow into the return line while sealing off the return fitting on the carb with your finger. This will pressurize the entire fuel system. In a quiet room you'll be able to hear any remaining leak in the fuel system. When you remove your finger from the fitting on the carb, fuel will eventually drip out indicating that the float chamber is primed full of fuel.

Return line restrictors - The round pump 44 Mikuni carbs were manufactured with an unrestricted return fitting on the carb. This causes them to return so much fuel to the tank that the fuel circuits can get starved for fuel. If you are using a round pump 44 Mikuni, be sure you have a restrictor jet in the return line whose inside diameter is no more than .030" (.75mm) All of the new generation square pump carbs have adequate "built in" return line restrictors.

Spark plugs - Most machines can safely use NGK #8 heat range spark plugs. However, in situations where you're trying to resolve a serious rich condition, it's best to do your preliminary tuning with #7 heat range plugs. After the tuning is done, however, return to the #8 heat range.

Pop off pressure - This term 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 round pump carbs should be between 8 - 12 psi. Initial pop off pressures on the square pump Mikuni and Kiehin carbs (unless otherwise specified) should be no less than 25 psi and no greater than 35 psi.

Dual carbs - Racing has certainly popularized dual carburetors. However having dual carbs is not necessarily the passport to big time horsepower. Dual carbs usually require a little more maintenance and on going adjustment. If putting up with that is not your cut of tea, you'll probably get all the performance you need, along with all the simplicity you want, from a good aftermarket single carb upgrade kit. If you've decided that aftermarket "duals" are for you, the pop off pressure between them should not vary more than one psi. Of equal importance, before doing any fine tuning, visually confirm that both throttle butterflies are closing completely and opening at the same instant. When setting the mixture screws, the settings should always be kept identical between the two carbs unless otherwise specified by the carb kit maker or your engine builder.

LOW SPEED ADJUSTMENT 0% to 35% throttle range - In 90% of all cases, the low speed mixture screw can be accurately set without riding the machine. Secure the boat on a submerged trailer or standing in about three feet of water. With the engine completely warmed up, set the carb up to a slightly higher than normal steady idle. Turn the low speed mixture screw in or out in 1/4 turn increments. As you get closer to the ideal setting, the engine rpm's will increase. If the idle speed is increased by this mixture adjustment, turn the idle speed adjustment screw down and continue the same process in 1/8 turn increments. At the ideal mixture setting, 1/8 turn in a richer or leaner direction will cause a very un-steady idle and cause the engine to die. To confirm your perfect low speed mixture setting, touch the kill button during idling...and then touch the start button about ten seconds later. The engine should restart instantly and idle steadily without touching the throttle.

Some race engines with heavy mid range fuel demands may eventually require as much as 1/4 turn richer adjustment from this ideal setting point, however the need for an over rich low speed mixture setting usually indicates an unacceptable lean condition in the mid range. Avoid running an over rich low speed mixture screw setting in an effort to cure a mid range hesitation (lean condition).

If you find that your ideal mixture setting is less than 1/2 turn out from bottoming, you should probably consider going to a slightly leaner (smaller number) internal low speed jet. If you find that your ideal setting is beyond 2 turns out, you should consider a slightly richer (larger number) internal low speed jet.

HIGH SPEED ADJUSTMENT 30% to 100% throttle range - The greatest fear of most racers is that of seizing an expensive engine as a result of running an excessively lean high speed fuel mixture. Some old racers contend that maximum horsepower is attained with a high speed fuel mixture that is at the brink of piston seizure. This fable is not true...and it never has been. Where high output pwc engines are concerned, the ideal high speed mixture is the richest setting that still permits strong acceleration up to peak rpm. With this in mind, it is always wise to start out slightly over rich and slowly lean the mixture out. If the high speed mixture is too rich, the peak rpm's will "sign off" prematurely. If the high speed mixture is too lean, you'll experience weak or "lazy" mid range acceleration.

You'll eventually find a narrow adjustment range where peak rpm operation seems unchanged. However, within this range, there should be a noticeable difference in mid to high range acceleration. Finding the setting, within this range, that gives the best "middle through high range acceleration" usually requires a couple of back to back 10-15 minute evaluation rides.

TRANSITION RANGE ADJUSTMENT 20% to 50% throttle range - The early style "round pump" 44 Mikuni is the only pwc carburetor ever manufactured with an external transition range adjustment screw. On the new generation square pump carbs, the manufacturers have abandoned the idea of this third adjustment screw because it caused so much confusion for the average watercraft owners and mechanics. The transition range of the new generation carbs is adjusted by changing the pop off pressure. These pop off pressure adjustments are made by way of various combinations of needle/seat sizes and float arm spring tensions.

In short terms, the transition circuit is richened by reducing pop off pressure with larger needle/seat sizes, as well as shorter or weaker tension float arm springs. The various combinations of larger needle valves and weaker springs result in a wide range of lower pop off pressures. Measuring the pop off pressure is how you determine whether you have made the transition circuit richer or leaner (higher pressures are leaner, lower pressures are richer) Most racing engines prefer very low pop off pressures because they demand so much fuel in the mid range. Unfortunately these low pop off pressures can often create an undesired (and nonadjustable) rich condition in the 0 - 30% range. This nonadjustable rich condition takes place at about 9 psi on the Mikuni square pump carb and about 13 psi on the Keihin. Running pop of pressures this low is not recommended. Ideally, you want to run the highest possible pop off pressure that permits "hesitation free" mid range throttle response.

AFTER TUNING - After you've found the best settings for your carb, make a record of it. By far the best place for this record is the inside of your hood, written in bold felt pen. Denote the final adjustment settings and the pop off pressure. If space permits, record your high and low speed jet sizes, as well as the day's temperature.

LATENT FUEL PRESSURIZATION - The powerful fuel pumps on the new generation Mikuni and Kiehin carbs has mandated a need for much higher pop off pressures than that of the earlier round pump carbs. Pop off pressures in the 25-35 psi range are very common. Without these high pop off pressures, the powerful fuel pumps would quickly flood the motor. All in all, this combination of a powerful pump working against a high pop off pressure is superior in all functional ways...except one.

When the engine is at rest, the pressure in the fuel system is virtually nil. As the engine is started, there is easily enough fuel delivery and fuel pressure to feed the meager needs of the low speed circuit for idling. However when the throttle of a "just started" engine is drawn slightly, a temporary lean condition of the transition circuit often causes the engine to hesitate badly or even stop altogether. This lean condition takes place because the fuel pump has not yet fully pressurized the float chamber of the carb. The 20% - 50% range transition circuit is solely dependent on full float chamber pressure in order to deliver it's fuel. Full float chamber pressure usually takes place after about 10 or 15 seconds of operation under a load. For a recreational rider who is slowly riding away from the beach , this momentary lack of low speed acceleration is no problem. However for the racer who must start an engine at the line, and operate it at little or no load before the start, this momentary lack of acceleration can be a big problem. If you are running dual carbs, this fuel pressurization problem can become even worse.

Unfortunately the only mechanical ways to reduce this hesitation problem is to reduce the fuel hose length, reduce the internal air volume of the fuel system, or slightly reduce the pop off pressure.

On most stock fuel systems the pump must draw the fuel through about three feet of hose from the gas tank to the fuel valve. After that, through another two feet of hose and a fuel filter. This long length of hose saps off a significant amount of fuel pressure and increases fuel pressurization times. To help resolve this on race boats, a direct line should be run from the reserve pickup on the gas tank to the carb. Along with this, it should be understood that the air space in the fuel system is the area that must be pressurized. If the air space is reduced by filling the gas tank, pressurization takes less time. Unfortunately many racers don't like the "nose heavy" handling that a full gas tank yields. For them a smaller gas tank is a wise solution.

The risk of inducing an over rich transition circuit mixture makes "pop off pressure reduction" an absolute last resort choice for solving the latent fuel pressurization that takes place in the starting area of a race.

By Harry Klemm -


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