2001 KX250 mods and performance

[Swimr2DaResQ]

Hey Fellas,

Just curious if anybody has or had a KX250 from '99-'02, and had some input and or advice on this bike.
I got the bike from a kid who was scared of it and did not know a thing about it. It's been a great bike so far with no major issues. I have added a FMF SST pipe and silencer (it came with a Gnarly pipe, now it is so much smoother and revs alot higher with more power up top), VForce 3 Reed system (best mod I've put on so far, much greater throttle response over stock!!!), NGK BR8EIX plug, I'm currently running Silkolene Racing 2T pre-mix in 91 octane gas with PJ1 Octane Plus at a ratio of 32:1, rebuilt Carb (Keihin Power Jet PWK38S Carburetor with K-TRIC?) with stock settings, other than needle in third from the top position, throttle response is quick and crisp throughout the powerband, plug looks good(slighty brown in color when checked after an all out run). I ride anywhere from Sealevel up to 2000ft, i estimate about 90 hours, + or - a few ,by word of mouth from the previous owner and the time i've put on the machine, it starts within two kicks, the compression always feels strong (but i don't have the tools to truly test that yet).

It runs good, but it feels a little weaker than before the Carb rebuild!!! 

I don't have any really big issues with it, I'm just looking to fine tune it and get the most out of the whole bike. I figured I'd start with the motor before I mess with Suspension and Chassis!

Any advice and/or input would be greatly appreciated

Thanks, Chad

2T's a Smokin', 4T's a Jokin' !!!!

[ford832]

Good bike.My first mod was always a twin air filter and boyesen dual stage reeds(glass).I usually end up dropping two sizes on the main and pilot with this set up but it's a very noticeable mod-power and economy both.

[eprovenzano]

I had a 99, it was a great bike, very reliable.  I added a 11oz flywheel weight.  It helped to reduce the wheel spin, and put the power to the ground.

Jetting can really change the power delivery.  When jetting, do one change at a time.  This way you will know exactly how the bike reacts to the change.

If you run it a little fat, it will give you a little more torque.  I try dropping the clip one notch to see if you like the power delivery, then go up one (from the current settings) to see how the power delivery feels, while checking the plug...

Let us know how it works out.

[Swimr2DaResQ]

Thanks fellas, the needle position will be the first thing i mess with, it will be a few weeks before i can do it, but i will let you know how it goes.
One other little thing i found, my subframe cracked. I plan on welding it, but i also looked at getting an aluminum sub frame. Has anyone ever used or have any comments on them?

[ford832]

Steel is perfect.Clean it,mig it,paint it and be done.Mine has an aluminum subframe but I'd prefer steel,especially after having to bend and tig it after my last crash.

[KXwestYZ]

I have a 2000 KX 250 and its the coolest bike I've ever been on. I got KTM SX 250 2004 recently but it didnt come close to the hellraising powerband the KX has so I sold the KTM last month, just didnt come close, it had a bit more lower torque but at expense of fun in the higher revs.. didnt see the point cos it takes all the fun away!

I ride mine on the road in Central London - creates a stir everywhere I go - just need to find a riding buddy cos im only person with a true MX 2 stroke on the roads here 

[Swimr2DaResQ]

Does anyone know if a 2003-05 kx 125 or 250 swingarm would fit a 2001 kx 250, are the dimesions the same? i know there is not much difference in the frame but the swingarm is different.

[Chokey]

Does anyone know if a 2003-05 kx 125 or 250 swingarm would fit a 2001 kx 250, are the dimesions the same? i know there is not much difference in the frame but the swingarm is different.

The swingarm can be made to fit, but it will require the '03-'05 axle and hub, as well as the caliper and caliper carrier. In '03 the axle increased in diameter to 25mm.

[Chokey]

Hey Fellas,

Just curious if anybody has or had a KX250 from '99-'02, and had some input and or advice on this bike.
I got the bike from a kid who was scared of it and did not know a thing about it. It's been a great bike so far with no major issues. I have added a FMF SST pipe and silencer (it came with a Gnarly pipe, now it is so much smoother and revs alot higher with more power up top), VForce 3 Reed system (best mod I've put on so far, much greater throttle response over stock!!!), NGK BR8EIX plug, I'm currently running Silkolene Racing 2T pre-mix in 91 octane gas with PJ1 Octane Plus at a ratio of 32:1, rebuilt Carb (Keihin Power Jet PWK38S Carburetor with K-TRIC?) with stock settings, other than needle in third from the top position, throttle response is quick and crisp throughout the powerband, plug looks good(slighty brown in color when checked after an all out run). I ride anywhere from Sealevel up to 2000ft, i estimate about 90 hours, + or - a few ,by word of mouth from the previous owner and the time i've put on the machine, it starts within two kicks, the compression always feels strong (but i don't have the tools to truly test that yet).

It runs good, but it feels a little weaker than before the Carb rebuild!!! 

I don't have any really big issues with it, I'm just looking to fine tune it and get the most out of the whole bike. I figured I'd start with the motor before I mess with Suspension and Chassis!

Any advice and/or input would be greatly appreciated

Thanks, Chad

2T's a Smokin', 4T's a Jokin' !!!!

It honestly sounds to me like it's top-end time.

Not trying to bash your mods, but you would get better results from a Rad Valve in place of the V-Force. And you couldn't pay me to run an SST pipe.

[Swimr2DaResQ]

alright that is good info, it sounds like it isn't really worth changing the swingarm out for a newer one.

I've had good reults with the v-force system, far better than stock, but i haven't used a rad valve system yet.
the sst pipe, i actually like, i had a gnarly pipe on before, the sst is smoother and still pulls harder and alot further on top. I might try a pro cicuit next or an fmf factory fatty, not sure yet!

I def need to replace the seals and gaskets, my seals are leaking oil from the top of the head, and i plan on rebuilding the exhaust valve(kips system) too.

[Chokey]

alright that is good info, it sounds like it isn't really worth changing the swingarm out for a newer one.

Nope, there would be no advantage.

I've had good reults with the v-force system, far better than stock, but i haven't used a rad valve system yet.

The V-Force will produce some improvement in throttle response, but no significant boost in actual power production. The Rad Valve on the other hand will produce tangible gains in  power production, provided you know how to jet. If you can't jet properly, there's no point to installing any mods anyway.

the sst pipe, i actually like, i had a gnarly pipe on before, the sst is smoother and still pulls harder and alot further on top. I might try a pro cicuit next or an fmf factory fatty, not sure yet!

My dislike of the SST is not performance related.

Originally written by Moto814

The FMF SST pipe is an amazing pipe.

Why is it amazing? Because in just about every case, it will work WORSE than the stock pipe that came on the bike.

The SST uses a restrictive reverse cone and pressure bleed to intensify the return wave and lower the initial flow out the stinger section of the pipe. What that in turn does is increase piston crown temperatures and make jetting difficult at best.

Now increased piston crown temperatures can be compensated for to some extent with richer jetting. But, by doing that you are defeating the purpose of the pipe, and negating any power gain you may have gotten from the pipe in the first place. Why are increased piston crown temperatures bad? Well, in some cases they are not. You see most bikes are jetted horribly rich from the factory, and raising the piston crown temps in this case will "fool" the engine into thinking it is jetted leaner than it is. Which will in some cases improve power. But in most cases, it will just cause slightly better throttle response and an increase in the spooge out the back of the pipe.

After all that you may be thinking: "well this doesn't sound so bad, why are they saying this pipe is no good?" Good question. Here's the answer. Because of the restrictive nature of the pressure bleed from the pipe, it does not evacuate as efficiently as it needs to. This combined with the piston crown temperature increase yeilds jetting that is too rich. So now you have a pipe that "fools" the engine into thinking it is leaner than it really is, but also makes it much richer than it was previously. Here in lies the second problem. It makes jetting a nightmare. If you jet it by feel, you will be jetting richer, and compounding the too rich condition even more, making the bike run like a slug. If you jet it leaner, you will be increasing piston crown temperatures even more resulting in an engine siezure. Catch-22 anyone?

If you are buying a pipe for your bike, get ANY other pipe. Or better yet, keep the stock pipe. The late model (1999-2005) KX250 pipes are pretty good stock, and provide a good low to mid powerband. If you want more over-rev, go with a Pro-Circuit Works or Platinum pipe. If you want more low end go with an FMF Gnarly, or Pro-Circuit Platinum2.

[Swimr2DaResQ]

Thanks for the info!

 I found a 03 swingarm on ebay as well 99-02 swingarms. just thought I would check it out. I really like this bike, and was looking to lighten it up a bit, i thought the newer swingarm was lighter and stronger, but regardless it would'nt be worth the effort to change it out.

I am going to try the ATF fluid now as well, i really don't like paying 8 bucks a quart for racing oil every other ride!

Do you have any other suggestions about this bike and or performance?
      Thanks again,
         Chad

[Chokey]

 thought the newer swingarm was lighter and stronger

It is, but not enough to be of any real significance.

 

Do you have any other suggestions about this bike and or performance?
      Thanks again,
         Chad

Give it a fresh top-end. You'll be surprised at the difference. The performance loss as the piston and rings wear is so gradual that you don't really notice it until you freshen up the engine and realize how much better it runs now. But peak performance starts to drop after as little as 10 hours.

[Swimr2DaResQ]

SST, mmmhhhhhh, Good to know!

Well, that certainly explains alot about the jetting issues I had, I think I will have to try another pipe!

 

[Swimr2DaResQ]

Does anyone know if the 03'-07' kx 250 plastics will fit the 99'-02' kx 250. I want to "pimp my ride," just a little face lift!