This is an article I found that may help someone out.
JETTING
By Greg Hall
Dirtwheels Magazine
Hot Horsepower Tips--April 2002
You think you did everything right. Bought the most expensive pipe. Paid extra for
that jet kit. Followed all the instructions to the letter. If you were lucky, the
machine just might actually perform better—except, of course, for that pesky
off-idle stumble or the giant flat spot in the midrange, or how about the severe top
end miss (must be the darn rev limiter!).
Experience tells me that almost every hop-up anybody has ever done to an ATV
has produced at least one point in the rev range that carburetion is considerably
worse than stock. Why? Because all you have is a piece of paper that gives you
recommended jetting settings. And unless you happen to be lucky enough to ride
in those same conditions, there will be some point in the rev range where you will
be too rich or too lean.
WHAT IS JETTING
Jetting is providing the engine with a combustible mixture. The ideal combustible
mixture ratio is 14.7 parts of air to one part of fuel, with the most power being
produced around 12-13:1. While a motor can (and will) operate on a mixture that
is considerably richer or leaner, power output falls off. If you happen to go leaner
and ride it hard, you may end up with an over-heated motor, or worse, a seizure.
Also be aware that carburetion is measured at throttle position settings. It has
nothing to do with engine rpm or transmission gears. So telling the pipe
manufacturer or Boss that it skips in third. but not fifth gear is totally useless
information.
Did you know that your fuel pre-mix ratio (two-strokes only!) affects your jetting?
A carburetor jet flows X amount of fuel and air at a given time. In that fuel is, say,
32 parts of fuel and 1 part of oil (32:1). If you change your pre-mix ratio to 20 to 1
because you are afraid of burning up your motor, all of a sudden the amount of
fuel has been decreased by 37.5 percent. And since it is the fuel and NOT the oil
that keeps your ring-ding cool, you run even leaner and hotter! Same theory
applies to four-strokes as well; the more fuel entering the engine, the cooler the
piston will be. The oil and water cooling systems are not designed to cool the
piston; only the little bit of fuel that is mixed with the incoming air charge prevents
your motor from seizing. ONLY after the heat has been transferred through the
piston to the rings and then to the cylinder, will the cooling system get the chance
to do its job.
WHY DO I HAVE TO REJET?
In a stock engine, the factory has spent a considerable amount of time and money
trying various jets and needles to come up with jetting that not only passes the
EPA (Environmental Protection Agency) regs, but allows the machine to be
operated at roughly factory rated output without overheating and blowing up.
When you, as an owner, change anything to do with the intake that would remove
factory built-in restrictions to air flow into the engine or, exhaust changes that
would do the same for air flow out of the engine, then you will need to rejet. Why?
A carburetor is designed with fixed size main and slow (pilot) jets. The jet needle
attached to the bottom of the slide is fixed at a certain height. Only the idle
mixture screw is adjustable. If you have increased air flow as outlined above, the
increased volume will still be mixed with the same amount of fuel as before,
resulting in a lean mixture. If you replace the main jet with a larger numbered jet,
the jet’s internal hole will be larger, thus flowing a greater quantity of fuel at
3/4-full throttle. If you raise the position of the slide’s jet needle by lowering the
jet needle clip, you are allowing more fuel to rise out of the needle jet at a given
part throttle position which is generally 1/4-3/4 open. If you replace the low speed
(pilot) jet with a larger numbered jet, the internal hole will be larger, thus flowing
more fuel at very small openings of 1/16-1/4 throttle.
I STILL FEEL I CAN JET BETTER THAN THE FACTORY CAN
Even if you popped for the extra expense of a jetting kit, don’t expect your jetting
to be "spot on" unless you are willing to experiment and try different jets. Why?
Say you install the main jet the jet kit recommends and it seems to run OK. Is it
truly the best for your machine in your riding conditions? It may not be, unless
you experiment by going up a jet size at a time until your machine exhibits a
stumble at full throttle, indicating a too rich mixture. Then by dropping back one
size you can be confident that now you have the correct jet for your machine in
your riding conditions.
The same thing should also be done with the other fixed jets of your carburetor
(jet needle and slow speed pilot jet.).
WORKING WITH INDIVIDUAL CARBURETOR CIRCUITS
So, how do you start? At the bottom. Then you jump to the top and work your
way down.
IDLE MIXTURE SCREW:
The idle mixture screw is the only externally adjustable carburetor jet available
and controls up to 1/8 throttle only. There are two types of idle mixture screws.
One type is called a fuel screw because it regulates the flow of fuel into the idle
circuit. This type of screw is located ahead of the carb’s slide tower (motor side)
and is most often found under the carb’s bore and upside-down directly ahead of
the carb’s float bowl. By turning the screw out you increase the amount of fuel that
is allowed to slip around the tapered needle and into the carb’s bore where it is
mixed with air that has snuck under the carb’s slide.
If the idle mixture adjustment screw is located behind the carb’s slide tower
(airbox side) then the adjusting needle regulates air flow into a fixed flow of fuel
intended for idle. By turning this screw inward you are reducing the air flow, thus
richening the idle mixture.
When the motor is up to operating temperature, set your idle speed screw to a
stable idle. Then use either your idle fuel or air screw to obtain a stable idle. Reset
the idle speed screw as necessary after obtaining the correct idle mixture.
MAIN JET
The main jet controls 3/4-full throttle only. Ideally you should start very rich
(large numbered jet) and test at full throttle. It should skip. If not then you are
not rich enough! Once you have your rich stumble, back off one size at a time until
full throttle operation results in normal operation. (Note: If your ATV runs faster at
3/4 throttle than full throttle you are definitely lean on the main!)
JET NEEDLE
The slide’s jet needle controls 1/4-3/4 throttle. It does this by passing upward
through the needle jet. The needle jet is a long brass tube that contains many
small holes in its sides that air passes through. Fuel from the float bowl enters this
air stream from the main jet and into the center of the needle jet where it mixes
with the air to create an emulsion. This mixture of fuel and air is then metered by
the height, taper and diameter of the jet needle as the emulsion passes upward
around the jet needle into the carb’s bore where it mixes with still more air to
(hopefully) arrive in the motor in a combustible fuel-to-air ratio.
If you have a soft hesitation, without a hard stumble, anywhere between 1/4 and
3/4 throttle, chances are your needle is lean, so raise the needle by lowering the
clip. Conversely, if you have a hard stumble, chances are the needle position is
rich, so lower the needle by raising the clip.
If you get very unlucky you might have to start playing with jet needle taper
which controls how fast the mixture increases as the jet needle is raised. This
would come into play if you were lean at 1/4 throttle, yet rich at 3/4 throttle. The
length of the needle comes into play here too. The diameter of the needle controls
how much fuel escapes around the needle while still inside the needle jet. The
larger the diameter of the straight section or "L" length, the leaner the mixture. Or
finally, the "L" length, which controls how much the slide rises before the tapered
part of the needle starts.
SLIDE CUT-A-WAY
The slide cut-a-way controls the amount of air allowed to pass under the slide at
1/8-1/4 throttle. It controls the transition from the low speed (pilot) jet to the
main jet-fed needle jet/jet needle. Replacing the slide with one that has a smaller
number (less cut-a-way) will decrease the amount of airflow under the slide at
1/8-1/4 throttle openings, thus creating a richer mixture at that throttle opening.
If you have a rich condition at 1/8-1/4 throttle and you can’t go any leaner, try a
smaller cut-a way.
But thankfully, jet needle taper, diameter, "L" length and slide cut-a-way are
usually not affected by most simple pipe/air filter modifications.
LOW SPEED (PILOT) JET
The low speed (pilot) jet controls fuel flow at 1/8-1/4 throttle. The low speed
(pilot) jet is usually not affected by most simple pipe/air filter modifications.
However, a slightly lean low speed (pilot) jet can raise havoc in the winter where
its fuel is added to the total mixture strength required to start. You may find going
one level up will help a winter cold start situation.
Finally your idle mixture is revisited if you have a deceleration backfire situation.
When you chop the throttle and use the motor to decelerate, if you get a stream
of backfires, try increasing your idle mixture strength 1/4 turn at a time until the
backfire goes away. Note: If you reach a point where your idle mixture is 4 turns
out (for fuel type screws, NOT air type screws), try going up one size on the slow
speed (pilot) jet and reset your idle mixture screw to 1-1/2 turns out and repeat
the process.
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Originally Posted by DirtBikeMagazine
