Crf250x best mods?

I have recently bought an 08 250x and I'm wondering what are the best mods to wake up the bike and make it run better without cutting airbox, aftermarket pipes. Any help is appreciated!

Without changing anything:

1. Check the jetting and see if it has the stock jetting or not. 

2. Modify the accel pump either with an O-ring or by buying a stiffer return spring.

3. I would at least consider taking the snorkel off the air box (two pop rivets) and possibly poking a few holes in the top of the air box.

4. Buy a smog block off kit if it has the smog pump on it....really won't "run" better, but you'll get no popping on decel then and drop about a 1lb off the bike.

5. Get the right springs for your weight, type of ridding, and ability.  You won't work as hard on the bike.



What does that accelerator pump spring Do? 

The accel pump gives you an extra shot of gas when you snap the throttle.  Prior to '09, the carb had a poor design for the linkage for the accel pump.    Here's the old and new designs side by side (click on the image to make it bigger):


 The black arm in the center is part of of the AP linkage.  In both cases, that arm rotates clockwise when you open the throttle, and goes counter-clockwise to return.

 In the old design, if you look carefully, the end of the black arm rests to the left of the adjustment screw, while in the new linkage, it's on the right side of the screw.     The screw is connected to the throttle and the black arm is connected to the accel pump by a rod.  When the black arm rotates, this is what gives you the squirt of gas.

So in the old design, it relied on the spring (which you can't see as it sits under the arm) to pull the arm long when the throttle opened (the screw moves away from the end of the arm).   This gives you a lag on timing of the accel pump and if the spring weakens at all, maybe a very weak squirt, resulting in a bog. 

In the new design, the screw pushes against the arm directly and the spring is used to return the arm, which is the exact opposite of the old setup.  Since the screw pushes directly against the arm, you end up with no lag in timing or a weak squirt.

So on the old design, you can do the "o-ring" mod, which is to place an o-ring around the arm and screw tying them together, or you can replace the spring that sits under the arm with a much stiffer one.   The spring is a better choice as sooner or later the o-ring will break, but an o-ring is far cheaper than a spring (spring is about $15 I think).   Some use safety wire as well or in place of the o-ring.  But wire breaks too, as that linkage sees constant usage.

Then of course you could always get a '09 or above carb, but that would be really expensive<g>


Edited by Jim Dettman

Mods I did so far, in order of importance in my opinion:

-Front and rear springs for my weight + correct sag - I'm 230 pounds, the way the bike handled after this change was drastically improved

-JD jetting

-Air box mod

-13 tooth front sprocket

-Tubliss with knobby tires for rocky area, AT81RC rear and Maxxis IT front - I ride a lot of gnarly rocky tight trails, and this drastically improved my ride quality and love the tire choice so far.

-Smog kit


By far, the 13 tooth front sprocket change for the money had a tremendous impact in the low range torque for tight trails. For 10$, everyone that owns an X should try this "upgrade". You do trade off about 5-6 km/h of top speed, but the low end torque gain is drastic. This will of course all depend on the type of terrain you ride. If you tend to ride a lot of slow single trail, give this a try.

The 2007 and newer bikes don't need the accel pump spring like the older bikes.

The #1 mod for those bikes is an R cam. I don't consider getting the jetting correct a mod, it is simply correcting a running condition that was intentionally made too lean to keep the EPA happy,

It all depends on where and how you ride. If you want MX power and can make use of it for your riding environment than go for it. If you ride more technical terrain then a different tune is appropriate.

I have a X and it has had a lot of R stuff on it, and when I started riding slower, gnarly, and more technical trails I began changing things so the bike now has X exhaust, different gearing, Stealhy flywheel, and  X fork valving. The CCC mods and JD Jetting have stayed.

OP, I know you mentioned no cutting the airbox, but I think that's the easiest and best first mod.  It significantly improvement for very little cost.  It creates more power and will even start easier.  It's rather absurd how small the stock snorkel is on the airbox.  Opening the airbox allows for more more air, so you need to re-jet. 

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