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Jetting article
submitted by site Admin
Midwestrider
The first thing I have
to say is stay away form
"jet kits" The reasons
why, will be explained
in this article. I will
first list the common
jets that you might have
to change. And then how
they will effect
performance and
reliability, then
finally how to tune your
ATV.
The Jets:
Pilot or Slow jet
This jet is a slotted
head jet that is located
in the float bowl and
next to the main jet. It
can be accessed by
removing the float bowl.
The main jet is a 6mm
hex head jet located in
the float bowl. It sits
on a tower. Is can be
accessed through the
little plug on the
bottom of the carb. How
to change the main jet
The main jet
The main jet controls
the a/f mixture form
about 3/4 to full
throttle. However the
main jet also affects
the amount of fuel that
the needle is allowed to
meter. The fuel is first
metered by the main jet
and then its up to the
needle to allow the fuel
to enter the intake.
The needle jet
The needle jet is a
tapered rod that is
lifted in and out of the
main jet. The needle is
attached to the slide.
And is located in the
carb body. It can be
accessed through the top
cover of the carb how to
change the needle
The needle jet mainly
controls the mid range
of the throttle. This
goes form about 1/4
throttle to about 3/4
throttle. The needle has
three different
characteristics. They
each affect different
part of the range. The
most common thing is the
clip postition. The clip
position refers to the
grooves cut in to the
top of the needle. The
different grooves allow
you to raise or lower
the needle, this make
the whole needle
throttle range richer or
leaner. If you were to
lower the clip postition
it would raise the
needle, this allows more
fuel making the mid
range of the throttle
richer. The opposite
would make the mid range
leaner. The diameter of
the needle refers to how
big the tip of the
needle is. This affects
the beginning of the
range for the needle,
about 1/4 throttle. If
the tip is bigger it
"plugs" the main jet a
little more. So it makes
it leaner. A smaller
needle tip lets more
fuel flow. so it makes
it richer. The taper.
The needle has a taper
about half way down.
Taper is there so has
the needle is pulled
bout 3/4 the way out of
the emulsion tube, It
can start add more fuel
for the range. It kind
of transitions the half
throttle range in to the
range were the main jet
picks up.
The Good Stuff - How to
Tune your ATV
MAIN JET
This is the first jet
you need to get correct.
This is because the main
jet feeds the needle.
Now their are a few ways
to determine the correct
jet. One way is to go by
feel. What you do is
find a place were you
can ride full throttle.
Go ride with that jet
and see how it feels.
Now change to the next
bigger jet, go ride and
see if that one makes
the quad quicker. Keep
going up until you feel
the power drop off and
then put the last jet
you had back in. If on
the first jet change you
feel the quad is slower,
you need to go down in
jets. Do the same thing
go down until the power
drops, then put the last
jet back in. The second
way to determine if the
jet is correct is to do
a plug chop. What you do
is warm up the quad, and
after it's warmed up put
in a brand new
sparkplug. Now start up
the quad and ride full
throttle 1st though at
least 4th gear. Next
pull in the clutch and
use the kill switch to
stop the motor, and
coast to a stop with the
motor off. Now where the
quad sits pull the plug
out and look at it.
There are a couple ways
of reading the plug. One
way is to look at the
white insulator that
holds the center
electrode. It should be
almost white with a
slight tan, yellow tint.
Another way is to look
at the white insulator
that holds the center
electrode. You need to
look way deep into the
reach. All the way down
to were the insulator
meets the body. If the
main is good you should
see a 1mm soot ring
around the insulator at
the base. You may have
to cut side of the plug
off to see it. Another
way is to look at the
center electrode itself.
Where the center
electrode meets the
insulator their should
be a small white line on
the electrode. Another
way to determine the
jetting is to have it
dyno tuned.
NEEDLE JET
The needle is setting is
often over looked. You
go about finding
the proper needle
setting by using the
seat of your pants. This
is similar to the main
but you use 1/4-3/4
throttle instead. Keep
going richer until it
starts to loose power
then go back to the last
clip position. Getting
the right needle setting
will improve reliability
and throttle response.
It improves reliability
because the 1/4- 3/4
range is where you do
most of the riding. It
improve the throttle
response by keeping the
a/f in the sweet spot
from 1/4 to 3/4 range.
PILOT JET
Last step, the
pilot jet. If the rpm's
stay up for a little bit
after you snap the
throttle or if the quad
wants to bog or cut out
as the throttle is
snapped the pilot is
lean. If its rich
it will be hard to start
or it will bog as the
throttle is opened. The
pilot screw is used to
fine tune the pilot
circuit.
Now only use keihin jets
because they are meant
for your quad and do not
require an adapter to
work. Stay away from
"jet kits" because they
give you a set of jets
to use and they give you
a general instruction
sheet that says to use
the same jets no matter
what brands you used for
mods, meaning they would
tell you to use the same
jet no matter who made
the pipe.
Some starting points for
jetting:
Stock
118mj
48pj
1 3/4 turns out on the
pilot screw
3rd clip stock needle
If you get a pipe and
take off the airbox lid
(or cut it to match the
hrc lid)
Nice gains
185mj
50pj
2 turn out on the pilot
screw
hrc needle 3rd or stock
needle 4 clip
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