By Greg Hall
Before changing jetting you should first
check for any intake leaks. Carb cleaner works great but
water will do. Start your quad or bike . Hold throttle at
stead idol spay the intake if there is a spike in the
RPM's, you will then need to fix the leak. Don't over torque
the intake bolts it will warp, making the leak worse.
MAKE ONE CHANGE AT
A TIME
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.
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.
fast
or last
Motorcycle
Carburetor Theory 101
Motorcycle carburetors look very complex, but with a
little theory, you can tune your bike for maximum
performance. All carburetors work under the basic principle
of atmospheric pressure. Atmospheric pressure is a powerful
force which exerts pressure on everything. It varies
slightly but is generally considered to be 15 pounds per
square inch (PSI). This means that atmospheric pressure is
pressing on everything at 15 PSI. By varying the atmospheric
pressure inside the engine and carburetor, we can change the
pressure and make fuel and air flow.
Atmospheric pressure will force high pressure to low
pressure. As the piston on a two stroke engine goes up (or
goes down on a four stroke engine), a low pressure is formed
inside the crankcase (above the piston on a four stroke).
This low pressure also causes a low pressure inside the
carburetor. Since the pressure is higher outside the engine
and carburetor, air will rush inside the carburetor and
engine until the pressure is equalized. The moving air going
through the carburetor will pick up fuel and mix with the
air.
Inside a carburetor is a venturi, fig 1. The
venturi is a restriction inside the carburetor that forces
air to speed up to get through. A river that suddenly
narrows can be used to illustrate what happens inside a
carb. The water in the river speeds up as it gets near the
narrowed shores and will get faster if the river narrows
even more. The same thing happens inside the carburetor. The
air that is speeding up will cause atmospheric pressure to
drop inside the carburetor. The faster the air moves, the
lower the pressure inside the carburetor.
FIG 1
Most motorcycle carburetor circuits
are governed by throttle position and not by engine
speed.There are five main metering systems inside most
motorcycle carburetors. These metering circuits overlap each
other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit
The pilot circuit has two adjustable parts, fig 2.
The pilot air screw and pilot jet. The air screw can be
located either near the back side of the carburetor or near
the front of the carburetor. If the screw is located near
the back, it regulates how much air enters the
circuit. If the screw is turned in, it reduces the amount of
air and richens the mixture. If it is turned out, it
opens the passage more and allows more air into the circuit
which results in a lean mixture. If the screw is
located near the front, it regulated fuel. The mixture will
be leaner if it is screwed in and richer if screwed out. If
the air screw has to be turned more than 2 turns out for
best idling, the next smaller size pilot jet will be needed.
FIG 2
The pilot jet is the part which
supplies most of the fuel at low throttle openings. It has a
small hole in it which restricts fuel flow though it.
Both the pilot air screw and pilot jet affects carburetion
from idle to around 1/4 throttle.
The slide valve affects carburetion between 1/8 thru
1/2 throttle. It especially affects it between 1/8 and
1/4 and has a lesser affect up to 1/2. The slides come in
various sizes and the size is determined by how much is
cutaway from the backside of it, fig 3. The larger
the cutaway, the leaner the mixture (since more air is
allowed through it) and the smaller the cutaway, the richer
the mixture will be. Throttle valves have numbers on them
that explains how much the cutaway is. If there is a 3
stamped into the slide, it has a 3.0mm cutaway, while a 1
will have a 1.0mm cutaway (which will be richer than a 3).
FIG 3
The jet needle and needle jet affects
carburetion from 1/4 thru 3/4 throttle. The jet needle is a
long tapered rod that controls how much fuel can be drawn
into the carburetor venturi. The thinner the taper, the
richer the mixture. The thicker the taper, the leaner the
mixture since the thicker taper will not allow as much fuel
into the venturi as a leaner one. The tapers are designed
very precisely to give different mixtures at different
throttle openings. Jet needles have grooves cut into the
top. A clip goes into one of these grooves and holds it from
falling or moving from the slide. The clip position can be
changed to make an engine run richer or leaner, fig 4.
If the engine needs to run leaner, the clip would be moved
higher. This will drop the needle farther down into the
needle jet and cause less fuel to flow past it. If the clip
is lowered, the jet needle is raised and the mixture will be
richer.
The needle jet is where the jet needle slides into.
Depending on the inside diameter of the needle jet, it will
affect the jet needle. The needle jet and jet needle work
together to control the fuel flow between the 1/8 thru 3/4
range. Most of the tuning for this range is done to the
jet needle, and not the needle jet.
FIG 4
The main jet controls fuel flow from
3/4 thru full throttle, fig 5. Once the throttle is
opened far enough, the jet needle is pulled high enough out
of the needle jet and the size of the hole in the main jet
begins to regulate fuel flow. Main jets have different size
holes in them and the bigger the hole, the more fuel that
will flow (and the richer the mixture). The higher the
number on the mainjet, the more fuel that can flow through
it and the richer the mixture.
FIG 5
The choke system is used to start
cold engines. Since the fuel in a cold engine is sticking to
the cylinder walls due to condensation, the mixture is too
lean for the engine to start. The choke system will add fuel
to the engine to compensate for the fuel that is stuck to
the cylinder walls. Once the engine is warmed up,
condensation is not a problem, and the choke is not needed.
The air/fuel mixture must be changes to meet the demands
of the needs of the engine. The ideal air/fuel ratio is 14.7
grams of air to 1 gram of fuel. This ideal ratio is only
achieved for a very short period while the engine is
running. Due to the incomplete vaporization of fuel at slow
speeds or the additional fuel required at high speeds, the
actual operational air/fuel ratio is usually richer.
Figure 6 shows the actual air/fuel ratio for any given
throttle opening.
FIG 6
Carburetor Jetting
Troubleshooting
Carburetor troubleshooting is simple once the basic
principles are known. The first step is to find where the
engine is running poorly, fig 7. It must be
remembered that carburetor jetting is determined by the
throttle position, not engine speed. If the engine is
having troubles at low rpm (idle to 1/4 throttle), the pilot
system or slide valve is the likely problem. If the engine
has problems between 1/4 and 3/4 throttle, the jet needle
and needle jet (most likely the jet needle) is likely the
problem. If the engine is running poorly at 3/4 to full
throttle, the main jet is the likely problem.
FIG 7
While jetting carburetors, place a
piece of tape on the throttle housing. Place another piece
of tape on the throttle grip and draw a line (while the
throttle is at idle) straight across from one piece of tape
to the other. When these two lines are lined up, the engine
will be idling. Now open the throttle to full throttle and
draw another line directly across from it on the throttle
housing. At this point, there should be two lines on the
throttle housing, and one on the throttle grip. Now find the
half-way point between both of the lines on the throttle
housing. Make a mark and this will show when the throttle is
at half throttle. Divide the spaces up even again until
idle, 1/4, 1/2, 3/4, and full throttle positions are known.
These lines will be used to quickly find the exact throttle
opening while jetting.
Clean the air filter and warm the bike up. Accelerate
through the gears until the throttle is at full throttle (a
slight uphill is the best place for this). After a few
seconds of full throttle running, quickly pull in the clutch
and stop the engine (Do not allow the engine to idle or
coast to a stop). Remove the spark plug and look at its
color. It should be a light tan color
(for more info on reading spark plugs click here).
If it's white, the air/fuel mixture is too lean and a bigger
main jet will have to be installed. If it's black or dark
brown, the air/fuel mixture is too rich and a smaller main
jet will have to be installed. While changing jets, change
them one size at a time, test run after each change, and
look at the plug color after each run.
After the main jet has been set, run the bike at half
throttle and check the plug color. If it's white, lower the
clip on the jet needle to richen the air/fuel mixture. If
it's dark brown or black, raise the clip to lean the
air/fuel mixture.
The pilot circuit can be adjusted while the bike is
idling and then test run. If the engine is running poorly
just off of idle, the pilot jet screw can be turned in or
out to change the air-fuel mixture. If the screw is in the
back of the carburetor, screwing it out will lean the
mixture while screwing it in will richen it. If the
adjustment screw is in the front of the carburetor, it will
be the opposite. If turning the screw between one and two
and a half doesn't have any affect, the pilot jet will have
to be replaced with either a larger or smaller one. While
adjusting the pilot screw, turn it 1/4 turn at a time and
test run the bike between adjustments. Adjust the pilot
circuit until the motorcycle runs cleanly off of idle with
no hesitations or bogs.
Altitude, Humidity, &
Air Temperature
Once the jetting is set and the bike is running good,
there are many factors that will change the performane of
the engine. Altitude, air temperature, and humidity are big
factors that will affect how an engine will run. Air density
increases as air gets colder. This means that there are more
oxygen molecules in the same space when the air is cold.
When the temerature drops, the engine will run leaner and
more fule will have to be added to compensate. When the air
temerature gets warmer, the engine will run richer and less
fuel will be needed. An engine that is jetted at 32š
fahrenheight may run poorly when the temperature reaches 90š
fahrenheight.
Altitude affects jetting since there are less air
molecules as altitude increases. A bike that runs good at
sea level will run rich at 10,000 ft due to the thinner air.
Humidy is how much moister is in the air. As humidy
increases, jetting will be richer. A bike that runs fins in
the mornings dry air may run rich as the day goes on and the
humidity increases.
Correction factors are sometimes used to find the correct
carburetor settings for changing temperatures and altitudes.
The chart in fig 8, shows a typical correction factor
chart. To use this chart, jet the carburetor and write down
the pilot and main jet sizes. Determine the correct air
temperature and follow the chart over to the right until the
correct elevation is found. Move straight down from this
point until the correct correction factor is found. Using
fig 8 as an example, the air temperature is 95š
Fahrenheit and the altitude is 3200 ft. The correction
factor will be 0.92. To find out the correction main and
pilot jets, multiple the correction factor and each jet
size. A main jet size of 350 would be multiplied by 0.92 and
the new main jet size would be a 322. A pilot jet size of 40
would be multiplied by 0.92 and the pilot jet size would be
36.8.
FIG 8
Correction factors can also be used
to find the correct settings for the needle jet, jet needle,
and air screw. Use the chart from fig 9 and determine
the correction factor. Then use the table below to determine
what to do with the needle jet, jet needle, and air screw.
| Needle Jet/Jet Needle/Air
Screw Correction Chart |
| Correction factor |
1.04 or above |
1.04-1.00 |
1.00-0.96 |
0.96-0.92 |
0.92 or below |
| Needle jet |
Two sizes larger |
One size larger |
Same size |
One size smaller |
Two sizes smaller |
| Jet needle setting |
Lower clip position |
Same |
Same |
Same |
Raise clip one position |
| Air screw opening |
One turn in |
1/2 turn in |
Same |
1/2 turn out |
One turn out |
