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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.
UNDERSTANDING SOME BACKGROUND
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.
BEFORE YOU ENTER THE WATER
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.