91 KDX 250 back together

[mgouras]

Got my bike put back together, and thought I'd share a pic. Because, who doesn't like to look at KDXs?
I put a new wiseco top end in, which I previsouly asked a question about. It's all good. compression is 180-190.
Some other things I did since picking up this $750 wonder last month (much with the help of the archives on here):
- I change jetting to 38 pilot, 160 main, top clip.
- repacked silencer (turbine core)
- New fork seals, and seal savers. Changed fluid to PJ1 5W.
- new hoses.
- new chain and sprockets
- new wheel, swingarm bearings. link bearings all looked good and tight on sleeves.
- cleaned power valve. That was a bit more troublesome than I expected.
- Fresh gearsaver tranny fluid
- Fresh 91 octane with amsoil mixed 40:1
- Maier hand guards
- tighten and (mostly) trued spokes
- new boyesen reeds
- new shifter
- Removed air snorkle, cleaned filter box and filter

I broke it in with three warm-up/cool-down cycles. Then I took it out for a very light shakedown run today. Overall, I was pleased with the results. It was about 35 degrees out today and it started first kick with choke on! And started first kick when warm. The plug seemed to be a light tan color, after hitting kill switch at about half throttle and checking it.
Ran pretty good. Was comfortable, and the motor was like a tractor in the snow-covered mud.
But I'm not sure the forks are right. They really seems to rebound awful slowly. They seemed OK putting around. But my baseline is non-existent, sine I have not ridden it before. The thing is, if you hold the brake and press down on the forks you can see them very slowly rebound. It almost seems if they sag a little low, too.
Things I have tried:
-Use factory manual method of releasing air in forks while it is sitting under its own weight
-Made sure the fork clamps are torqued to the proper level
-loosened up fork clamps, axle and brake caliper and pushed down repeatedly on forks to see if anything was sticking. re-tighened.
Is it normal and nothing to worry about? Due to the cold weather? Any insight would be nice.
This is my first bike in 15 years, when I rode a '90 YZ250 WR. I did borrow a friend's KTM300EXC from a friend a couple years ago for a day of trail riding -- and that's the only riding I've done in the meantime!

 

[karlp]

I'd look again at those lower triple tree bolts. It is easy to overtighten them. If you loosen them and the rebound speeds up, it would point to them being over tightened.

I believe the '95-2006 KDX 200/220 forks will go right on? You'd have to find everything from the triple trees down.

 

[dirt bike dave]

At least part of why your rebounds seems so slow and the front of the bike sags so much is your bike has very soft fork springs for its weight. FWIW, I would not give too much weight to how the bike feels when you are pushing it up and down in your driveway. How it works on the trail is far more important!

I had a '91 KDX250 and the stock fork settings are not good at all. As with other KDX's, the bike has too much high speed compression damping and too soft a spring. This is fine on the street (KDX's are dual sported in many parts of the world) but bad off road, as you are too soft for jumps and big whoops (springs too soft), but too stiff on rocks and roots (too much high speed compression damping).

On mine, I revalved the fork with a Race Tech gold valve, put in stiffer (but not too stiff springs, .40 kg/mm, I think). I also had no pre-load on the new springs, as this help the front end dive down for tight corners (the stock steering geometry is very slow). This greatly improved the bike.

I really enjoyed mine, but the bike takes a little more effort to set up properly than most others. The stock jetting and forks are especially bad. It is also hard to get your weight forward, but this can be improved by removing some foam from the front portion of the seat. After some tuning, the motor was very smooth and already has a heavy flywheel weight. You can get more low end by using a thin head gasket from a '88 KX250, and it will still run fine on pump premium.

If you want to replace the front lower fork guards and can't find a listing for KDX250, I think the ones from a KLX250 or 300 will fit just fine.

 

[mgouras]

Can you add a couple of shims at the top of the spring to make it stiffer, or is this just a silly idea that would cause the fork to bind?

 

[dirt bike dave]

The problem with additional pre-load is it makes the fork more resistant to initial movement. If you ride trails with lots of rocks and roots, this is not good on a stock valved KDX250 front end. It will help the front end ride higher, but the bike is already slow steering. Better to have stiffer springs and less preload, IMO.

If your budget does not permit new fork springs, you can make your old ones stiffer by cutting off a few coils. Let's say you have 40 coils per spring now. If you remove 2 your spring will be about 5% stiffer. Don't cut off too much or you will get coil bind! Measure the gap between two coils and multiply this by the number of gaps. Say 8mm x 40 gaps = 320 mm. If your fork has 300 mm of travel and you remove 2 gaps of 8mm each, your spring could compress as small as 304mm, so you would not have coil bind. These are ball park figures used to illustrate the point - your travel and springs may differ.

Some fork basics are as follows:

Oil weight mainly effects rebound damping (heavier oil = slower rebound). With 5w, you are probably using the right oil.

Oil level has a big effect on bottoming resistance and fork stiffness in the last part of your travel. If you are bottoming too easily, raise the oil level. If you never bottom and the forks are stiff at the last part of the travel, lower oil level.

 

[mgouras]

I doubt I would just chop the spring. I will probably wait until I get around to new springs.
But, if you cut the spring, would you not have to shim up the same amount you cut off? Wouldn't the forks ride 8mm lower if you cut 8mm off?

 

[dirt bike dave]

I doubt I would just chop the spring. I will probably wait until I get around to new springs.
But, if you cut the spring, would you not have to shim up the same amount you cut off? Wouldn't the forks ride 8mm lower if you cut 8mm off?

Yes, if you cut the spring you would have to add a pre-load spacer if you wanted to keep the same amount of pre-load.

 

[glad2ride]

What brand fork seals did you use? No matter what brand, they will need some time to properly break in.

 

[mgouras]

They are kawasaki seals. Good to know on break-in period. Thanks.

 

[glad2ride]

No problem. That is a nice looking KDX250, by the way!

Don't worry about adding spacers to the fork springs. They are just way too soft to be set up properly in a modern way. They need to be much stiffer. Just skip spacers and get some springs. After you get the correct springs, THEN worry about the valving.

 

[Tom68]

KDX 250 Forks

Had similar problems with my forks.Springs were down to 460mm, supposed to be over 500mm, fitted length is 455mm.Couldn't get accurate enough heavier springs and I think they may exceed stress allowances from factory, so I went the old fashioned route and added a T piece and schrader valve.Running between 8 and 12 PSI with 5W oil now set at 80mm, couldn't be happier with it.

 

[mgouras]

Again, good info and interesting solution. I'm going to poke around and see what's available in the way of springs for this thing.

 

[badfish5446]

Nice Work, the bike looks great. I just recently bought a '93 Kdx 250 and am still learning. I am still trying to figure out how to take off the seat to access the air cleaner.... LOL

 

[dirt bike dave]

Nice Work, the bike looks great. I just recently bought a '93 Kdx 250 and am still learning. I am still trying to figure out how to take off the seat to access the air cleaner.... LOL

Must be your first real dirt bike, lol.

Besides the two bolts at the rear of the seat (one on each side), the front of the seat has a slot that slides over a screw on the tank. You need to push the front of the seat down and slide the seat backwards. :nod:

 

[badfish5446]

Actually it is LOL. Thanks im gonna try it out tomorrow ;-)