Need info on tweaking a KDX 250 1991 Model E1

Rejet - the stock jetting is horribly rich.  Also, a carb slide with 1mm more cutaway than stock will help (or grind approx. 1mm off the bottom of your stock slide) 

On my old one, the KIPS valves when fully open did not fit flush with the floor of the exhaust port, so I used a dremel tool to remove some material from the bottom of the side KIPS valves - this added some top end power but did not hurt low end.

Use a thin head gasket from an '88 KX250. 

The stock pipe and silencer are very heavy and have a low/mid range hit.  The power will be smoothed and more top end power added with an FMF pipe, and you can modify an aftermarket KDX200 silencer from an '89 to '94 to fit.

Revalve the front forks and put stiffer springs in.  The stock forks have way too much high speed damping and not enough low speed damping, and too soft springs. With the stiffer springs (approx 0.40kg/mm) run no preload to improve turning.  Fixing the forks makes a world of difference on this bike.

Put a 13t countershaft sprocket on in place of the 14t.

Cut some foam from the front of the seat to improve the riding poisition.

Grind some material off the steering stop to improve tight turning.

Shorten the chain and run the wheel all the way forward.

There are lots of little things to do to improve the bike.  The motor has a very heavy flywheel stock and can to made to run pretty strong.  The main problem with the bike is it is very heavy for a 250 two stroke. 

With a little preparation, the KDX250 can be a very fun, reliable bike.  I liked mine more than my old 200's. 

The main advantage of an aftermarket silencer is the weight savings, but it also helps smooth the bike's 'hit' in the mid range.  I think you can still buy an FMF pipe for the KDX250, but no one makes an aftermarket silencer to my knowledge.  My buddy had modified an aftermarket KDX200 silencer to fit his '92 KDX250, and he gave it to me when he sold his bike. 

If you search for some posts from Matt90GT, he will have info on jetting and other updates.  I sold my '91 about 4 years ago, but it served me well.  The bike got bad reviews in the press and never sold well in the states, but basically it is a decent bike that suffered from poor jetting, slow steering, poor fork settings and excess weight.  You can easily fix all the problems except the weight.  Replacing the pipe & silencer will shave a few pounds off, but if you are serious about trimming some fat you can also remove the lights, odometer, and kickstand.

BTW, the rear fender supports are heavy, but if you remove them both, the fender will crack, especially if you use the stock tool bag.  You can remove just one fender support if you carry your tools in a fanny pack, and the fender will last a long time.

Like other KDXs, the chain guide is weak and can easily break or bend in ruts or rocks - the Fredette Racing Products chain guide is far superior. 

 

To use an aftermarket (much lighter) KDX200 silencer on the 250, he welded on a short bracket near the middle of the '89 to '94 KDX200 silencer.  Combined with the existing rear silencer mounting bracket, the new bracket allowed the silencer to bolt to the frame in the same two locations used by the stock 250 silencer.  He also fabricated a small bit of tubing to join the 200 silencer to the 250 pipe.

Our KDX250's over here in the US were not street legal.  They did not need keys to start, nor did they have oil injection like the KDX-SR. 

BTW, both my '91 and and my friend's '92 had fuel tanks that measured 3.6 gallons (over 13.7 liters), where the manual says 3.3 gallons.  One of the complaints the magazines had about the bike back in the day was the wide tank/riding position, but if you favor long rides the extra capacity/long range is great.  Also, with the heavy flywheel you don't have to worry about spending money on a flywheel weight, that's for certain!

More than most bikes, the KDX250 really benefits from some minor modifications and set up.  If you take a little time getting it dialed in, it is a fine bike for an affordable price. 

Matt90GT
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Joined DRN: May 2002
Location: Oregon




12-20-2003 04:09 AM
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The 250s require more work to setup, but are faster than a 200 once done.

You need a 88 KX head gasket , .26" thick
remove the silencer
new pipe
V force II reeds
Carb - #6 slide, 1367 needle, and 38/155-158 jetting.

Once you do that, the motor rips. Especially with a port job. Like any used bike, watch the bearings and linkage for slop.

The bike is heavier, but you can remove about 10 pounds by swapping out to a 90-91 KX 250 rear fender and ditching the metal supports there. The forks are sensative since they only have compression adjustment. You can swap to a 91-94 KX fork easily if you get it complete. Even use your stock clamps. That helps a ton right there!

They have good points, large fuel tank, WR gears 5sp, and a 70w lighting coil. I have seen most ones in good shape go for around $2k lately, but like said watch what you get. When I got mine it cost about $700 to get the rear bearings, axles, pivot bolts and new unitrak link since the entire rear end was gone.

Originally posted by 89kdx200rdr
Matt90GT
Registered


Joined DRN: May 2002
Location: Oregon




12-20-2003 04:09 AM
--------------------------------------------------------------------------------

The 250s require more work to setup, but are faster than a 200 once done.

You need a 88 KX head gasket , .26" thick
remove the silencer
new pipe
V force II reeds
Carb - #6 slide, 1367 needle, and 38/155-158 jetting.

Once you do that, the motor rips. Especially with a port job. Like any used bike, watch the bearings and linkage for slop.

The bike is heavier, but you can remove about 10 pounds by swapping out to a 90-91 KX 250 rear fender and ditching the metal supports there. The forks are sensative since they only have compression adjustment. You can swap to a 91-94 KX fork easily if you get it complete. Even use your stock clamps. That helps a ton right there!

They have good points, large fuel tank, WR gears 5sp, and a 70w lighting coil. I have seen most ones in good shape go for around $2k lately, but like said watch what you get. When I got mine it cost about $700 to get the rear bearings, axles, pivot bolts and new unitrak link since the entire rear end was gone.

Click to expand...

I would take that carefully! That is for the US models! I dont know what will or will not transfer over to the SR models!

I also have a 91 KDX 250 - Seems very hard to obtain parts in the UK as most bike shops just want to sell you a new 4 stroke!

Anyway after some searching I can obtain almost anything and have gone for a DEP rear silencer and DEP front pipe. Which incidently is a KX250 89-93 version. I then went for a Wiseco piston, rings etc, mild port job and good old KIPS decarbonisation. This has made the bike wheelie in 1st, 2nd & 3rd just by twisting the throttle - no need to drop the clutch!

I purchased a Boysen RAD valve but think this was a total waste of money @ £150 :0(

It dedpends what you want from the bike really but for me it makes a great weekend toy that gets very muddy :0) And if I drop it, which I have many times, so what it does not matter. I paid £1000 for it and could have paid much much more for a decent KX220 or even a new 4 stroke!