argclh6670

Member
Jan 27, 2013
37
0
Hey people, I'm a noob to this site. I just came by a kdx 250 for my kid. As his skill developes, I'd like to mod it up for him. Presently its jetted pretty clean, I more concerned with suspension mods at the time as he's a bigger kid and he's always pushing it in the woods. Any n all suggestions appreciated. Kx fork swap n such. All the posts I read are saying premix. At 40 or 50/1. I's there a downfall to running at 32:1.
)
I have a complete frontend off a 96kx250 that I'm sure I can make work. Anybody else have input from their personal experiences?
 
Last edited:

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
I really enjoyed my '91, after getting it set up right.

The bike was not dialed in well from the factory. The jetting and the forks are the biggest problem areas, but there are lots of other tips and tricks.

Use the search feature and look for some old threads by blackduc748 Some of his old posts had detailed answers to the same questions you are asking.

Oh, and no downside to running 32:1 except for spending more money on oil.
 

jeffcooper

Member
Nov 21, 2007
15
0
Definately swap those forks - the difference to the bike after is like night and day, and in my case the bike just felt better all through - even the power felt better because I was riding more confidently. However, I've now fitted the engine to a 95 KX250 frame (but used the 18" rear wheel from the KDX) and to be honest, it's like you suddenly gain about 100% in rider ability - in fact I now spend time thinking how bad I could have hurt myself if the old, extremely heavy bike had landed on me !!
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
On mine, I did gold valves in the forks, put in stiffer springs with no preload, raised the forks so about 3/8" was above the top triple clamp, and ran a WER steering damper. Had a gold valve in the shock, too, but as with other KDX models, the stock shock is better than the fork. The forks have too soft a spring, too much high speed compression damping and too soft slow speed damping, so they basically suck everywhere.


The stock seat/tank junction is not good for getting forward. I shaved the seat foam down at the front of the seat and that made a nice improvement.

Run a good front tire to help the front end bite as much as possible.

BTW, the manual says the tank is 3.2 gallons, but my buddy and I both measured ours at 3.6 gallons. Plenty big.

I found with a 1 tooth smaller front sprocket I could take 2 links out of chain and run the wheel all the way forward in the adjustments.

The bike has a heavy flywheel stock, so you won't be needing a flywheel weight.

A thin head gasket from an '88 KX250 will give you more compression and low end power.

The steering geometry is pretty slow, so anything you can do to improve that will help. My buddy actually cut his frame and pulled in the steering angle, and that helped. He's qualified to do that kind of work, but for 99.9% of the owners, it's not worth the trouble.

You can trim a little off the steering stops to get the wheel to turn more (don't go too far).

On mine, I trimmed the side KIPS valves. When open, the bottom of the KIPS valve was much higher than the exhaust port. I dremeled the KIPS valve so when open it was flush with the exhaust port.

The stock pipe and silencer weigh a ton. I ran an FMF pipe, and modified an aftermarket slilencer for an '89 - '94 KDX200 to fit. You can remove the rear fender supports to save a few pounds, but don't run a fender mounted tool bag if you do this.

The bike is heavy, for sure. Wet, no gas and set up for trail it will probably be about 255 lbs. I had mine dual sported, and it was about 260 with the stock pipe/silencer on it.
 

argclh6670

Member
Jan 27, 2013
37
0
ill be doing a topend on it shortly, putting the 88 gasket, ill also look into the kips thing. Although im NOT totally clear on what you mean on this mod. What improvement did you get from it? /also, i have a complete front end off a 96 kx250 that im thinking bout swapping stems and running, pretty sure that'll work? good info dave, jeff. thanks.

i also have bare roller frame from that 96 kx250 wonder if it would be worth while to contemplate swapping over to it? IDK....any thoughts on that one??
 

argclh6670

Member
Jan 27, 2013
37
0
Jeff,

beings you have done the swap, was it hard? any making of special mounts? do you know if the 96 frame would even work? thanks. tony
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
argclh6670 said:
ill also look into the kips thing. Although im NOT totally clear on what you mean on this mod. What improvement did you get from it? /also, i have a complete front end off a 96 kx250 that im thinking bout swapping stems and running, pretty sure that'll work? good info dave, jeff. thanks.

i also have bare roller frame from that 96 kx250 wonder if it would be worth while to contemplate swapping over to it? IDK....any thoughts on that one??

On the KDX250, the KIPS valve has two 'barrels', one on each side of the main exhaust port. When the side barrels rotate to the open postion, exhaust gas flows through them. On mine and my buddies, when the 'barrels' were fully open, the bottom of the opening in the barrel was not flush with the bottom of the exhaust port. Each of them was at least 2 mm above the bottom of the port. So I removed some material from the opening in the barrel to make the opening bigger. Hard to say if it made any improvement. In theory, it helped the top end power.

On mine, there was also a whole bunch of casting slag in the intake tract of the cylinder, so I ground that off, too.

I had a Boyesen Rad valve on it, also.

After leaning the jetting out (a lot) and running a BR8E7 or BR7ES plug, I was pretty happy with the motor for trail work. Considering how pig rich it was jetted stock, the motor responded well to the minor mods. A six speed tranny would have been nice for dual sport riding, but the 5 speed box was fine for trail riding.

As for the frame swap, I think it is very much worth considering, especially if you don't need the big KDX tank.
 

jeffcooper

Member
Nov 21, 2007
15
0
Tony - I think the 96 frame is identical to the 95, so it should work, it's very easy to get the engine out and slip it in, so I recommend you give that a go just to check first! The swap wasn't hard though - you have to cut some off the swing-arm to fit each side of the engine, and the sleeve bushes to match, and I had to drill out the engine castings, about 1 or 2 mm to get the thicker swing-arm pivot bolt through - the carb/airbox connection is a little bit shorter as well, although it fit, you just have to try hard !

Other than that, everything fit like it was made for it, although of course you lose the speedo cable drive, so go digital, or forget it! oh, I did have to open out one of the bolt holes on the engine top mounts. Cable routing underneath the tank is a tad tricky as well - and you would need the KX radiators, even though you can mate them up with the KDX engine with a bit of adjustment.

Bottom line is that I'm no engineer - it was all done with basic tools in the shed at the bottom of the garden, so it couldn't have been hard. One thing I do miss though on the much, much, much lighter, better handling bike is the side stand !!!!!
 
Last edited:

argclh6670

Member
Jan 27, 2013
37
0
Tony - I think the 96 frame is identical to the 95, so it should work, it's very easy to get the engine out and slip it in, so I recommend you give that a go just to check first! The swap wasn't hard though - you have to cut some off the swing-arm to fit each side of the engine, and the sleeve bushes to match, and I had to drill out the engine castings, about 1 or 2 mm to get the thicker swing-arm pivot bolt through - the carb/airbox connection is a little bit shorter as well, although it fit, you just have to try hard !

Other than that, everything fit like it was made for it, although of course you lose the speedo cable drive, so go digital, or forget it! oh, I did have to open out one of the bolt holes on the engine top mounts. Cable routing underneath the tank is a tad tricky as well - and you would need the KX radiators, even though you can mate them up with the KDX engine with a bit of adjustment.

Bottom line is that I'm no engineer - it was all done with basic tools in the shed at the bottom of the garden, so it couldn't have been hard. One thing I do miss though on the much, much, much lighter, better handling bike is the side stand !!!!!
Hey jeff, im finally starting the swap over to the kx frame...got all parts, motor sitting in frame.....if u can remember when you cut the swingarm , did the cut go into the hollow portion of your swingarm?? I havent cut mine yet.....just tryn to get alil more info.
 
Top Bottom