Honda CRF250X - Jetting

August 26, 2016 version


 My Modifications 







'08 250X/R carburetor changes 
'07 250X/R carburetor changes 
Jetting comments 
Intake and exhaust changes 
Jetting related 
Parts and prices 

'08 250X/R carburetor changes

The '08 250X carbs (49-state and CA) are identical to the '07 250X carbs except for one change:
Fuel screw (pilot screw) 49-state - 1 3/4 turns out, CA - 2 turns out

The '08 250R carb is identical to the '07 250R carb except for the following:
Main jet - 188; jet needle - NMGU; fuel screw (pilot screw) - 1 7/8 turns out



'07 250X/R carburetor changes

Honda made some significant changes to the '07 250X and 250R carburetors.

The '07 250X CA carb is different than the 49-state model (as in past years), and there is a new designation for the 49-state model that specifies it's for the Canadian model 250X also.

Both '07 250X carbs have a new jet needle (aka, just plain 'needle') holder assembly that holds the needle. The new needle has a different top end to mount in the new needle holder assembly. The assembly includes the needle holder, spring, spring seat, and clip. The entire assembly, including the captive needle, screws into the top of the slide using a torx driver. The 49-state and Canadian model needle has only one clip position; the CA model needle has two clip positions (normal and high altitude). I have not yet looked for alternate needles, but I'm thinking jetting just got a bit harder. Most 4-stroke riding is done 'on the needle', and raising and lowering the needle is critical to getting good carb performance.

The '07 250X 49-state carb has a new designation for the leak jet - Jet, leak (ACCP) - 99108-KSE-0600. From the part number, I'm guessing it's a size 60.

Both 49-state and CA carbs also have a new accelerator pump and linkage for improved throttle response. I'll try to get pictures and details ASAP.

The '07 250R carb has the same new needle assembly and new needle design that the X carbs do. The needle only has five clip positions (previously there were 7).

The carb also has a new accelerator pump and linkage for improved throttle response.

There is now a set plate in the bottom of the float bowl that looks like it's there to prevent fuel from sloshing around too much. Perhaps it helps keep fuel supplied to the main jet when landing off jumps. (There is no set plate in the X float bowl.)

The bottom line for all three '07 carbs - all JD and older Honda needles will work in the new '07 carbs.

To install JD and older Honda needles:
* remove the carb top,
* remove the '07 needle holder assembly and captive needle from the carb,
* remove the '07 needle from the needle holder assembly,
* drop the replacement needle with mounted clip directly into the carb slide,
* screw the '07 needle holder assembly in and re-install the carb cover.

Other jetting-related change:

Previously ('04-'06), the California model 250X had a secondary air injection system, which introduces filtered air into the exhaust gases in the exhaust port. There are extra hoses and an air pump under the fuel tank.

This system is on ALL USA 250X models beginning with the 2007 model year.



Jetting comments

Jetting is a topic about which almost every rider has an opinion. Here is my very brief and condensed opinion of how to approach the problem of jetting.

There are four parts of the carb that are normally adjusted to get the fuel/air mix to the correct values for maximum engine performance: pilot jet, fuel screw, needle clip, and main jet.

On four strokes, we ride almost 90% of the time "on the needle", meaning that we ride with the throttle open to the position where jetting is controlled primarily by the needle. Of course, the pilot and main jets are always contributing at all throttle openings, but the primary controlling factors are needle diameter, taper, and clip position for 90% of typical riding. Just think about how you ride and you'll see that this is true. No one idles the bike while riding (fuel screw and pilot jet), nor do we actually ride at full throttle (main jet) or anywhere near (unless you're Stefan Everts or Ricky Carmichael). Typically, only needle clip position is changed during jetting.

Step 1 - I usually use a jetting chart to get the pilot and main jets in the ballpark and set the fuel screw as the first step in jetting, but there is no Honda-provided jetting chart for the 250X. The 250X bikes are jetted very lean from the factory. For lower elevation riding, you might want to go larger a few steps on the main and perhaps 42 on the pilot.

Step 2 - Once you have changed the pilot and main jets to get the best performance you can, needle clip position should be adjusted. Go up or down on the needle clip according to your local riding conditions. Raise the clip to lean the jetting and lower the clip to richen the jetting. Try going a step or two beyond the clip position you feel is optimal to insure that you have the best clip position. You may have to go back to the pilot and/or main jets and change them as you get the carb into optimal tune.

Once you get optimal performance, the needle clip adjustment is the only adjustment you should be making for moderately varying altitude and temperature changes. For small altitude and temperature changes, adjust the fuel screw. To make adjustments easier, I suggest you replace the stock fuel screw with an aftermarket extended fuel screw such as one from KoubaLink or Zip-Ty. You should be adjusting the fuel screw to compensate for small changes in altitude and temperature almost on a per-ride basis. The extended fuel screw will make this adjustment easier.

It is a fact that the pilot and main jets are easier to get to on the 250X carb than the needle. Nevertheless, you should perfect the technique of adjusting the needle clip position because of its critical importance to optimal jetting.

That's it! You're done.

I've provided a bit more information below to fill out the picture, but really, only the foregoing is critical to jetting.
Honda provides different needles in the 250 R, X, and X-CA carbs and also optional needles for the R. Several aftermarket carburetor companies also provide a wide range of needles. James Dean (jetting guru on ThumperTalk) sells jetting kits for the CRF carbs. These kits have two needles and several main jets and have very complete jetting instructions. If you're discriminating enough, you may decide you need to change your needle for better performance, although I have never felt the need to do so. Or, you may decide that the JD kit and instructions are the easiest way to go; many riders do and are very happy with the results. Regardless of which needle you end up with, you still have seven clip positions, and you need to select the one that gives optimal performance.

The slow air jet (pilot air jet), starter jet, and leak jet are also part of your carb's adjustments, but these jets are changed so rarely that for all practical purposes, you can ignore them. Some riders feel that changing the leak jet is important, but I feel that Honda has chosen leak jet sizes that appear to be close to optimal for the 250 R, X, and X-CA carbs. If you are changing your X to CCC specs, you may feel the need to try different leak jets.

The accelerator pump affects jetting by squirting fuel into the carb throat, towards the engine, just under the rising slide as you twist the throttle rapidly from idle or small throttle openings towards full wide open. Some riders feel that adjustments to this component are necessary, but if it is working properly, you can ignore the pump. Do test the pump to see if it is working properly (see Testing the pump) and keep the external pump mechanism clean, so it operates properly.

The carb floats also affect jetting by insuring the correct fuel level in the float bowl. Some riders prefer to adjust the floats so there is less fuel in the float bowl (larger float level value, lower float height), especially for high altitude riding. I deviate from stock about 1mm lower for my over 8,000' riding. Only do this adjustment after all other jetting elements are adjusted for optimal performance.

Other parts on your bike that affect jetting include the exhaust header pipe, muffler, exhaust diffuser, air box intake baffle, air box opening, air filter screen, air filter, and air filter oil. As you change any of these items, you may also need to adjust your jetting for optimal performance.

Lastly, the throttle position sensor affects the ignition advance curve, and it must be adjusted to the correct position. Check the TPS at least once during your initial jetting sessions, and recheck it yearly.



Intake and exhaust changes

If you remove the air snorkel from the top of the 250X air box and/or remove or drill out the diffuser pipe in the muffler, you will need to richen the jetting to compensate for the reduced air flow restrictions. This also applies if you change the exhaust system to an aftermarket system which breathes better than stock. The "Closed Course Competition Modifications" bulletin that Honda released describes what jetting to use if you perform all of their suggested modifications. If you only do a few modifications, then you need to make smaller changes than described in the bulletin.

Here's what James Dean (jetting guru on ThumperTalk) has to say about intake and exhaust changes:

Forum "CRF250R/X"
Subject "Real X versus R Comparison - Honest!"

The stock bike's power would not satisfy me for long. There was not enough torque for hill climbs and wheelies in the middle gears from my perspective. Open the air box up along with rejetting and the mid-range comes alive. The most restrictive part on the bike is the intake snorkel and the #130 main jet. The CRF250R by comparison has a #160 main and most R riders increase that to a #165 (or more).

Removing the exhaust baffle seemed to be as loud as a stock CRF250R, maybe louder. The tone has a sharper "popping" that seems more annoying than the lower tone of the 250R exhaust. Secondly, more of the power gains came from opening up the intake side and rejetting. The exhaust baffle is not as restrictive as the WR250F, and will be a useful muffler for lots of 250X owners.

I agree with him: opening up the intake and rejetting gives more power than opening up the exhaust. I opened up the air box on my 250X, but not as much as Honda recommends for the Closed Course Competition Modifications. I removed the screen from the air filter holder. I put in a 42 pilot jet, raised the needle one clip, and put in a bigger main jet. I spent an hour dialing in the fuel screw, riding and fiddling. It helped to have the extended fuel screw; I got mine from KoubaLink when I got my lowering link. Don't forget, I ride at 8,000' and up, so your numbers may be different.

I also drilled the inboard end of the exhaust diffuser because I had done the same to my CRF230F and I liked how it improved performance without making the bike too loud. It did the same for the 250X.

Try opening up the intake and rejetting before you remove or modify the exhaust diffuser.



Jetting related

DaleL reports: "When I was testing out the carb I found that my Throttle Position Sensor (TPS), according to the shop manual, was out of spec from the factory. Owners may want to check that out if they're making modifications. I re-positioned the TPS to put the idle resistance into spec."

1/10/05 - I made a disturbing discovery today. I have three 250X fuel screws: stock Honda, KoubaLink, and Q. The stock Honda and KoubaLink screws measured exactly the same everywhere, except the KoubaLink has a longer shaft and a T pin on the end.

The disturbing discovery was that the Q fuel screw had a smaller diameter tip than the other two. The tip I'm talking about is the very small pointy tip. The tips were all tapered, but the Q fuel screw was uniformly smaller in diameter. The tip is what actually controls the fuel flow. With a smaller diameter tip, the Q fuel screw will have to be screwed in further than the stock fuel screw to get the same flow.

If you're having trouble screwing in your aftermarket fuel screw and/or getting it to jet correctly trying to match other riders' settings, compare that fuel screw to the stocker to see if it's different.

(I'm not mentioning who the Q fuel screw is made by (it's not Zip-Ty). It may not be as critical as I suspect, and I don't want to run any company down based on my own findings. I'll leave it up to individual riders to check their own fuel screws. The letter Q is not related to the fuel screw manufacturer's name. I was using the letter X, but that was too confusing e.g. 250X.)


Parts and prices

Service Honda prices for Leak Jets (11/23/06)

Description Part number

Price (each)

Jet, leak (#55) - stock 250X 49-state - '04, '05, '06
                          stock 250X CA - '07
                          stock 250R - '06


Jet, leak (ACCP) - stock 250X 49-state - '07 99108-KSE-0600


Jet, leak (#65) - stock 250X CA - '04, '05, '06 99108-KSC-0650


Jet, leak (#70) - stock 250R - '04, '05, '07 99108-KRN-0700


Service Honda prices for Optional Leak Jets

Jet, leak (#40) 99108-KRN-0400


Jet, leak (#45) 99108-KRN-0450


Jet, leak (#50) 99108-KRN-0500


Jet, leak (#60) 99108-???-0600


Jet, leak (#75) 99108-KRN-0750


Jet, leak (#80) 99108-KRN-0800


Service Honda prices for Jet Needle Sets; aka just plain 'needles'

Needle Set, Jet (NCVT) - stock 250X 49-state - '04, '05, '06 16234-MEN-671


Needle Set, Jet (NCYU) - stock 250X CA - '04, '05, '06 16203-KSC-771


Needle Set, Jet (NCYR) - stock 250R - '04 16203-MEN-671


Needle Set, Jet (NCYQ) - stock 250R - '05 16202-MEN-671


Needle Set, Jet (NHFS) - stock 250R - '06 16203-KRN-851


Needle Set, Jet (NCYS) - CCC Mod 16204-MEN-671


Service Honda prices for other carb parts

Gasket set, carb - R/X all years
Contains carb o-rings (7) and rubber gaskets (3), including fuel screw o-ring (see next)


Screw set A - R/X all years
Fuel screw, spring, washer, and o-ring


Screw set, jet needle - R/X all years
Needle holder - screw, spring, shaped washer


Jet, slow (40) 99103-440-0400


Jet, slow (42) 99103-440-0420



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