How-To Change Jets and Needles in the Honda CRF450X Keihin FCR Carb


You 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.


Head and Valve Service for Honda CRF OEM Parts


The 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.


Terms Defined

The 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.



Since 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.


Changing the Jets

  1. Turn off the petcock.
  2. The 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.
  3. Be prepared to catch the fuel from the float when you remove the float drain bolt. Remove the float drain bolt.
  4. With 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.
  5. The 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.
  6. Once you've installed the new jets, replace the float drain bolt.
  7. Rotate 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 job.

Changing the Needle

  1. Unfortunately, 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.
  2. Once 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.
  3. Remove the throttle slide cover by removing the two allen head screws.
  4. Once 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.
  5. With some small needle nose pliers, you can extract the needle from the slide.
  6. Put 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.
  7. Now set the new needle's clip position.
  8. Insert the need into the slide.
  9. Replace the needle retainer.
  10. Replace the slide cover.
  11. If you rotated the carb for better access, rotate the carb back into place and tighten air boot clamps.

That's 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 situation.

CRF's Only "How-To" By CRF's Only, July 2005


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