Hot Horsepower Tips
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
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
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:
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.
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!)
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
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.
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
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.
One final note; reading about how to jet will not make you "good"
at jetting. And asking someone a thousand miles away why your machine
skips in third gear won’t get you the answers you seek. Even Boss can’t
help you when you write him asking "I just bought an XYZ pipe. What jet
do I need?"
Only hands-on, trial and error experience can solve your jetting
problem. So go purchase a handful of jets and get your hands dirty! You
are out five big ones for that pipe and jet kit, and now have a
hobbling pile. What are YOU going to do about it?