They are built off the 90-91 KX250 bikes. They have a 43mm inverted fork, dual disc brakes,KIPPS valve motor in them.
If you read a lot about them, you will find that a lot of places dont like them. They say overweight, sluggish, wide, no top end, etc. Personally my bike is far from stock, so I could not tell you what a stock KDX250 is.
A few things between the 91-94 model years:
91 Forks springs are .34kg, rear shock is 5.0
The 92-94 is .35 in the front and 4.9 in the rear.
The 91 uses a different odometer cable that is screwed in on both ends. The 92-94s use a set screw at the axle end.
You will want to check out my page for info on my bike and where parts come from:
http://mlbobbitt.home.attbi.com/kdx250/1991kdx250.html
Things to look for? Well with any used bike, expect to change all the fluids including the forks. Plan to replace the front and rear wheel bearings. I had to replace the unitrak link and all the rear suspension bearings so check those for play. The lower shock bearing is especially weak on the Kawasakis.
I had problems with the fork seals and fork dust wipers that I solved by installing some seal savers:
http://www.sealsavers.com . Also expect to grease the headstem bearings.
Other than that, usual 2 stroke motor checks.
Some of the things that I like about the bike is that it is light. Okay, compared to my XR600 I came off of and the 87 CR250 that weighed the same. The perimeter frame is great! Easy to replace the top end and the tank is HUGE on the bike stock. Once you find out where the parts come from, they are easy to get aftermarket units. got to like the 2 stroke with a headlight, always a bonus for later afterwork rides! They do come with a 70w stator stock so it runs the Dual Twinns headlight great in series. The forks are 43mm so they are the same size springs as the XR400s and Xr600s and lots of other bikes out there. So you can get a lot of used springs as takeouts from these bikes that are stiffer than your stockers. WR gears, 5 sp.
Cons, they could use a bit more steering travel. You can grind the stops down to help that for sure. With the kind of riding I do, 5 weight oil and adjusting the clickers on the both ends can compensate for most situations. If you are going to MX it, expect to increase the springs rates. The seals have been a bit hard to seal up but that is it.
Now my bike has a bit of mods. KX cylinder that is ported and polished, milled head (have to torn it down to confirm) <A href="http://www.dirtrider.net/cgi-bin/clickpro/click.cgi?id=21" target="_blank"><b>FMF</b></a> <A href="http://www.dirtrider.net/cgi-bin/clickpro/click.cgi?id=21" target="_blank"><b><u>pipe</u></b></a>, silencer and <A href="http://www.dirtrider.net/cgi-bin/clickpro/click.cgi?id=21" target="_blank"><b>FMF</b></a> torque reeds. The bike has a ton of torque and you ride a lot of time off the <A href="http://www.dirtrider.net/cgi-bin/clickpro/click.cgi?id=21" target="_blank"><b><u>pipe</u></b></a> in the real tight woods. I can easily outrun friends on KDX200 but when it gets really tight you all slow down to about the same pace. The bike is stable, easy to wheelie and works great for the riding I do. Personally I would love to pick up another unit for a spare mount! The bike tops out at 72mph with a 13/50 sprocket combo. Plenty fast! Much better than the CR250 close ratio box for the enduro scene personally.
Here are a few links that I have found around on the bike:
http://www.geocities.com/Baja/1685/1991250.html
http://showcase.netins.net/web/watsondesign/kdxpage.html
http://groups.msn.com/MyPicsandLife/shoebox.msnw?action=ShowPhoto&PhotoID=2&pgmarket=en-ca
If you have more specific questions, email me. From what I have read a stock KDX250 may not be a good mount. Do some motor work like install the thin KX headgasket for the bike, <A href="http://www.dirtrider.net/cgi-bin/clickpro/click.cgi?id=21" target="_blank"><b><u>pipe</u></b></a> and silencer and it will for sure make it a different bike! The suspension may need to be addressed also depending on your weight and riding style. The suspension is plush, but where I ride that is what you need.