250EX Performance FAQ thread- EDITED 1)/24/08
Theres alot of repetitive threads asking the same info over and over again. To cut down on answering the same questions all the time, and differing answers to further confuse everyone, here's a FAQ to clear everything up.
To find what you're looking for, simply look below this paragraph, and with luck you'll find a answer relating to a question you have. If you find something that is incorrect, inaccurate, or have additional info on anything 250-related, PM me. I dont log on here every day but I check my PMs semi-regularly so I will do my best to keep this up to date.
Table of Contents
1. Bolt-on Engine modifications w/ pricing
2. Top Speeds
3-11. Exhaust/Jetting info/Recommendations
12&13. Advanced Engine Modification (Cams, Valvesprings, etc)
14.Oils
15-19. Suspension
20. Carburetor Swaps
PLEASE NOTE, All price ranges are approximations. I am not a vendor so I cant guarantee you will find what you are looking for at the price I specify.
1. I have ____ to spend on my 250. What would be the best bang for the buck bolt-on modifications?
Go-Fast Stuff
UNI filter/airbox mods: 30 bucks + a few bucks for a jet or two
Exhaust: if money is tight, get a slip-on, but I would recommend the full thing. 150-450 bucks + a few bucks for jets
Airbox delete: Free + a few bucks to rejet
Carb swap: Free-200 bucks + a few bucks for jets
Tires and Wheels: 200-400 bucks.
Go Far Stuff
Nerf Bars: 200 bucks
2. How fast will my [Insert year here] 250ex with [Insert modifications here] go in 5th gear?
22" tires, stock rev limiter: 51-52 mph.
22" tires, +1000 rev limiter: 58-61 mph.
Recons will be slightly lower.
3. What pipe is the best for the 250EX?
There is no dynos proving gains with one pipe over another on a 250ex. I did see a MBRP exhaust improve peak numbers by 1.5 HP over stock some time ago. That said, most choose exhausts because of price or fit and finish. My Pro Circuit T-4 is 5 years old and has stood the test of time. FMF and DG are generally considered lower-end exhausts due to poor craftsmanship. Bottom line, dont believe all the hype you hear, buy an exhaust because of sound levels, and how well it is fabricated.
4.Do i need to rejet after installing a aftermarket air filter and/or aftermarket exhaust?
YES! Even in stock form Hondas are tuned in such a way that they run lean (not enough fuel to mix with the air) for the sake of emissions.
5. How do I rejet my carb?
Here is a crash course on how to rejet your carburetor:
An engine is like a air pump. the pump (engine) works based off of three basic ingredients- air, fuel, and spark. For basic purposes, spark usually doesn't need to be adjusted regardless of mods (it could benefit a litttle bit but thats getting pretty technical). that said, that leaves us with two ingredients, air and fuel.
For the engine to run properly, a certain amount of air needs to mix with the given amount of fuel flowing into the motor. this mixture is called your Air to Fuel ratio- commonly abbreviated as A/F or AFR. for best gas mileage, the ideal AFR is 14.7:1 , but if you're trying to make maximum power, you want roughly 12:1.
So let's say you're running lean (not enough fuel to mix with the air) , that would mean you have a high (example: 17:1) AFR. not good for the motor, you'll experiencing overheating, backfiring, poor performance.
A rich mixture will lead to bogging, poor performance, etc. This will lead to a low AFR (e.g 10:1 or so). It means too much fuel being mixed with the air.
So the whole point of rejetting, is to provide enough fuel to appropriately mix with the air coming in. stuff like airbox mods, exhausts, aftermarket air filters, all that stuff will increase the airflow coming into the engine. so you need to add more fuel. Rejetting does this.
2001-2005 250EX's only. 2006 and on I believe are setup slightly different, but the process is similiar.
1) Make sure your bike is clean prior. Turn Gas line to OFF
2) Pop the seat off
3) Remove tabs off left side plastic (left side when sitting on bike)
4) Remove tabs off airbox brackets
5) loosen clamps to snorkel and carb
6) remove crankcase breather tube (major PITA if you havent done it before)
7) Yank airbox out
8) remove 2 nuts holding carb to intake manifold
9) flip carb upside down ( expect a small amount of fuel to drip out)
10) unscrew the 3 screws holding the float bowl (bottom half of carb, or when upside down, top half) on
11) OK smack dab in the middle of the carb (still holding upside down) is the main jet on top with the needle jet) the other orifice is the pilot jet.
12) Swap whatever you need to :-d
For the e-clip, take a peek at this photo, credit to TedWilley for making this... 
Note: I have been told that the 2006-2007 250's DO NOT have an adjustable e-clip. I cannot verify this, however.
Jetting Recommendations:
NOTE: ALL of these recommendations are assuming sea level elevation and room temperature (70 degrees Fahrenheit). These recommendations are with Honda/Keihin/K&N jet sizes, NOT dynojet. For dynojet, others have reported to me that the jet numbering runs about 4 lower than the keihin/K&N numbers.(CREDIT GOES TO JDUB03, thank you 
)
Stock exhaust, stock airbox lid, stock filter: 95 is stock, bump the main up to a 98.
Stock exhaust, stock airbox lid on, UNI/K&N filter: 100 main, e-clip on the jet needle put 1 notch further down (4th of 6 settings)
Stock exhaust, no airbox lid, UNI/K&N filter: 102 main, same e-clip setting
Any full spark-arrested exhaust (Sparks, HMF, Pro Circuit), airbox lid on, UNI/K&N filter: 105 main jet, e-clip in the same position as above
Full exhaust, No airbox lid, UNI/K&N filter: 108 or 110 main jet, e-clip should be good in the 4th position.
Full exhaust w/ NO spark arrestor (run at your own risk), No lid, UNI/K&N filter: 110 or 112 main jet, e-clip should be fine in the 4th position.
No airbox, no snorkel, UNI/K&N, full exhaust w/ no arrestor, etc: 120 main, e-clip in 5th groove.
6. How come you didn't make any recommendations as far as the pilot jet goes?
I run a full exhaust with no arrestor or quiet core, no snorkel/airbox, and i still run a 38 pilot. Unless you have major motorwork or run in extreme cold don't bother.
7. What if i dont ride in cold weather or have motorwork and my idle still sucks?
If you are having idle/off idle problems you need to play with the fuel screw and/or your idle adjustment.
8. I used your jetting recommendations during spring/summer and my quad was running awesome! Now it's cold out and it runs like crap. What gives?
In short, your quad is now running lean again thanks to the change in temperature (and as a result , air density). A good and easy to memorize rule of thumb is that you should go 1 size bigger on the main for every 20 degrees colder than the temperature in which the bike was tuned, and 1 size smaller for every 20 degrees hotter. Or, if you feel like taking the easy way out, if you were running no lid during the summer, just throw the lid back on for the winter. Usually, you'll be just about right for the average 30 degree winter day assuming you were spot on with no lid on a 70-80 degree day.
9. My exhaust came with different jetting recommendations than the ones you suggested.
Good for you. Have fun running a 125 main in a stock 250 w/ a slip on exhaust.
10. What about air filters? Is there one better than another?
Yes and no. Some filters are better at filtering dirt and sand, while not flowing quite as well, and vice versa. I can say with confidence from my own experience that I have been 100% satisfied with running UNI filters. I run one on my 250, my dad's Scrambler, my old Z50, as well as my modified V8 Jeep. I ride in everything from mud to sugar sand, and they've held up and filtered it all. It all comes down to personal preference basically.
11. What about running a foam pre-filter on a K&N?
Good for filtering, bad for performance and your wallet. In addition to buying both , this means you need filter oil to properly oil both filters. It is a good idea if you run in the dunes for example and are going to a mudbog, or someplace that you wouldn't normally ride at.
12. Bolt-on mods are for sissies. I want real power!
1.Web Camshaft
2. Port/Polish
3. JE .020 piston
or, skip all of the above and slap a turbo on like me 
13. My valves float at high RPM. Is there such a thing as HD valvesprings for the 250EX?
Yes, R/D valvespring makes both Titanium and Chromoly Valvesprings for the 250ex. The Ti is 136 dollars, while the Chromo is 76. These springs would be ideal if you have a top-end oriented build, or have a cam/rev box.
14. What is the best motor oil for the 250ex?
I am not going to say one oil is better than another, because I know someone will get their panties in a wad about it. I've ran GN4, Amsoil synthetic, and ELF synthetic, and i found ELF to be the best bang for the buck motor oil thus far. Amsoil might be marginally better, but the difference between the two was negligible at best and the ELF was 25% cheaper. Synthetic is always better than conventional oil. Shell Rotella oil in both synthetic and non-synthetic form is another great oil for not much money.
Please be aware that synthetics with friction modifiers will make your clutch slip.
15. What about suspension? Who makes suspension for a 250ex?
Works and Elka both make front and rear shocks for the 250. Herrmann Racing and ASR both make +2 a-arms, as well.
16. Works and Elka shocks are just too stinkin' expensive. Is there any other options?
Suspension isn't cheap- but you get what you pay for. Although they will not handle as well as a quality set of aftermarket shocks and extended a-arms there are some cheaper solutions:
For the front suspension: 300ex shocks (Recon's only?), Diamond J Widening kit w/ DVX/400ex/300ex front shocks (OEM or aftermarket)
For the rear suspension: Banshee front shock, 300ex rear shock (all years w/ the use of 2 washers), 2001 Raptor rear shock
17.My 250 is pretty tippy in the corners. What can I do to make it a bit more stable?
1) Get rid of those rear balloons and get some real tires. Demands may vary, but for gravel/trails/grass I recommend the Razr.
2) This is expensive, but +2 a-arms/shocks/axle.
3) Wheel spacers. Expect lots of bent axles/tie rods/hub damage if you like jumps.
18. Who makes an axle for the 250? Can I switch them out myself?
Durablue makes one. Last time I checked, it was available through either Rocky Mountain ATV or Chaparell for 289.99.
And YES, an axle swap is a DIY job. It is easy but takes alot of time. You'll need a few seals/bearings , but nothing crazy. A OEM axle swap to replace a bent OEM axle (this includes seals, etc) ran me about $160 in parts from Service Honda. Now that i know what i'm doing, I could probably take a bent one out and install a new one in an hour.
Its been a while, but from what I remember this is how you replace the axle.
1) Make sure the quad is clean and free of any gunk, mud, or dirt that could contaminate bearings/seals.
2) Remove BOTH rear wheels. You'll need to remove the hubs- straighten up the cotter pin used to hold the castle nut in place, remove them, and once they are out, remove the castle nuts. If you are gentle with the cotter pins you can reuse them, BUT you are doing so at your own risk.
3) Remove the hubs.
4) Disconnect the rear brake cable.
5) Looking at the gear case from behind the quad, remove all the bolts holding the rear drum brake in place on the right side. Open it and inspect it for unusual wear and clean any debris that may have made its way in there.
6) It might be before you remove the drum brake, but at some point you have to remove the locknut holding the axle. You'll need a axle locknut tool- I got mine at the Honda dealer for 10-15 bucks. I connected a breaker bar to this tool and then slipped about 5 ft of pipe over it, and was able to break it loose.
7) Remove axle
8) This is the part where my mind gets a bit fuzzy, LOL. See my note below.
NOTE: I REALLY recommend getting the Honda Service manual for the 250ex axle swap. I used it, and didn't really know what I was getting myself into at the time and sure enough, I did it with no problem. If I ever have to swap out another axle, I will post a full write-up. The above instructions are, at best, guidelines for what you'll need to do.
19.Who makes extended A-arms for the 250?
ASR and Herrman Racings both make +2 a-arms. ASR offers 3 different sets, with different materials and more adjustability.
20. I want/need more performance, but I dont feel like rebuilding the top end or even tearing it apart if everything internally is fine. I already have intake/ full exhaust/rejetted carb, what should I do?
The best (or most noticeable) modification you can do given the above situation would be to upgrade to a bigger carburetor. A bigger carb means more air, and more air mixed with more fuel means more power! The below carbs will fit all years of 250ex's with minimal modifications:
NOTE: I have not done the carb swap yet. The only thing I can verify is that they will fit onto the intake manifold. The few that have done the swap have mentioned that the stock throttle cable does not fit on top of the XR carb. Based on TAINO RACING's experience, Motion Pro cable Part # 02-0472 will work, if you cut the cable down to approximately the 250ex cable's length, then resolder the piston back onto the cable. I'll figure it out in the next month or two here (Posted April 2009)
ATC200x carb- 24mm
XR200 carb- 26mm (all years EXCEPT the dual carb model)
DG carb for ATC200X/XR200- 27mm(?)
The following carbs will work with some modification:
300ex carb - all years
You'll need to rejet accordingly.
This will be an ongoing FAQ thread, I have more info to put down..[
Respect is earned, never given.
2002 250ex- SOLD.
Creator of the 250EX Performance FAQ. For 250ex headwork and various porting , please email me at flyinryanporting@gmail.com
Reply With Quote
You covered pretty much everything, but I would like to add in something. It makes it 10x easier to work on the 250 with either a Clymers manual or a Honda shop manual. From what I can tell, Clymers tells more on how to take apart the whole bike and put it back together, but doesn't go into as much detail on little things about the engine such as oil pressure. Where (from what I hear) the Honda Shop manual tells how to take it apart and put it back together, plus all the little engine details. I have a Clymers and it's more than enough for a backyard mechanic. It thouroughly explains on how to adjust the valves (and tells the clearance levels), shows wiring diagrams, gives a lot of electical information, and like I said before, it basically tells you how to take everything apart and put it back together. I've replaced my axle using it, replaced axle bearings, rear drums, adjusted valves, and a few other things with it. So if you do a lot of work on your quad yourself, I would definatly reccomend either a Clymers Manual or Honda Shop manual.
)
Oils
The best thing to do , if its gas that Sta-bil cant revive, is just take it to the local dump/recycling center, they will probably know what to do with it.