Change Jets and Needles in the Honda CRF450X Keihin
won't have your new CRF450X for long before someone
suggests "rejetting your carb" for increased
performance. If you've never rejetted a carb this
probably sounds very intimidating. Not so. It's actually
a very simple task that anyone can perform with a
little instruction. Now, selecting the correct jets
and needle to install, that's the hard part. More
on this later. While this how-to was conducted on
a CRF450X the process is the same on all Keihin FCR
carbs used in Honda CRF models.
jets are located at the bottom of the carb, while
the needle is located at the top of the carb. Simplistically
speaking, jets determine the maximum amount of fuel
that can be delivered into the airstream. The needle
meters the fuel based on the throttle position. The
size of the orifice in the jet determines the maximum
fuel flow. The larger the orifice the more potential
fuel flow. The needle is found in the throttle slide,
which allows it to move up and down as the throttle
is opened and closed. The needle itself is tapered
which, along with the throttle position, determines
the amount of fuel being metering into the airstream.
term "jets' typically refers to the main jet
and slow jet. The slow jet is also referred to the
pilot jet. The needle position determines how high
the needle sits in the throttle slide. This is often
referred to the "clip position." This can
be very confusing as raising the clip position actually
lowers the needle in the slide. While raising the
needle means lowering the clip position. Often you'll
hear someone suggest the needle needs to be in the
second position. Typically this means, the second
position from the top.
you'll be drain fuel from the fuel line make sure
you are working in a well ventilated area that is
void of any ignition source. It's always a good idea
to have a clean bike whenever you are wrenching. In
this case, it is more than a good idea, it's mandatory.
You'll be exposing the internals of the carb to the
surroundings. Any dirt that falls into the carb will
absolutely have a negative impact on performance.
Turn off the petcock.
easiest way to access the jets is to rotate the
carb clockwise from the left side of the bike. This
is accomplished by loosening the air boot clamps
on both sides of the carb. Once they are lose you
can easily rotate the carb far enough to access
the float drain bolt.
prepared to catch the fuel from the float when you
remove the float drain bolt. Remove the float drain
the float drain bolt removed, you can easily see
the main jet. The slow jet is more difficult to
see and photograph. You can remove and replace the
main jet with an 8mm socket.
slow jet requires a flat blade screwdriver to replace.
The slow jet will be located further forward and
higher in the carb. Be very careful with fitting
the screwdriver and make sure it's narrow as to
not damage surrounding components or surfaces.
you've installed the new jets, replace the float
the carb back into its original position. Use the
tabs in the air boots to correctly position the
carb. Tighten the air boot clamps to finish the
there's no easy way to access the top of the carb
to replace the needle. You'll have to remove the
seat and fuel tank to gain access. The seat is easily
removed by removing the two seat bolts at the rear
of the seat. The fuel tank is more involved. Refer
to your shop manual for detailed instructions. Basically,
you'll need to remove the fuel line from the petcock
on the carb side (single line side), remove the
petcock mounting bolt, remove the top fuel tank
bolt, remove the tank strap at the bottom of the
tank, and remove the radiator shroud bolts that
attach the shrouds to the radiators. Once these
are removed these you'll be able to remove the tank
with the shrouds still attached. Be careful with
the petcock as it's a very tight fit between the
frame and valve cover.
the fuel tank is removed, you'll be able to see
the top of the carb. You may wish to loosen the
air boot clamps to rotate the carb as needed. Also,
I find it helpful to remove the cable hanger on
the frame as shown in the picture.
the throttle slide cover by removing the two allen
the cover is removed you can loosen the needle retainer
screw. You do this with an allen wrench. It might
be helpful to use the throttle to raise the slide
up in the carb to gain better access.
some small needle nose pliers, you can extract the
needle from the slide.
the removed needle in a zip lock bag that identifies
exactly the needle inside. Store in a place that
won't cause damage to the needle.
set the new needle's clip position.
the need into the slide.
the needle retainer.
the slide cover.
you rotated the carb for better access, rotate the
carb back into place and tighten air boot clamps.
it! Famous last words I know. However, don't be intimidated
if you've never done this before. Read this a couple
of times, read your shop manual and go for it! The
hard part is determining which jets and needle to
use. JD Jetting has taken the guess work out of this
by hands-on research. You'll find JD Jetting kits
in our online catalog for the Honda CRF450R, CRF450X,
CRF250R, CRF250X, and CRF150R. These kits include needles and
jets for hot to cold climates and for altitudes ranging
from 0-12,000 feet. The included instructions provide
details on which needles and jets to use for every
Only "How-To" By CRF's Only, July 2005