KDX250 info

Discussion specific to the 1991 - 1994 KDX250 sold in the USA
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argclh6670
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KDX250 info

Post by argclh6670 »

#1
Figured some good info on the bikes would be a great sticky.

The KDX250 is a lot like the WR250 and RMX250 of the years. They are an enduro model MX bike from the factory. They suffer from poor jetting, engine performance and suspension. The factory added a XR style rear change adjuster, kick stand, larger tank, Wide Ratio tranny and lighting stator to the KX motor make up the mods to make it a KDX model.

To get the bike into peak performance, you will have to start with the motor, then jetting, then suspension.

Where to start your modifications:

]freebies:
1) Remove the airbox snorkle from the lid and cut the lid open to maximize air volume intake.
2) put the needle in the second notch from the top (carb will still be jetted to rich)
3) set the sag on the rear suspension

now you will find that the bike still has carb issues, performance issues and suspension issues. So you will be putting some money into the bike to make it into what I can become.

]money mods:
1) KX250 thin headgasketfrom 1988 (.26 vs .46 in thickness)
2) Rejet the carb -- 38 slow, 158 main, #6 slide, R1367 needle 3rd clip. This will bring the bike closer to the KX spec.
** I ran 50:1 amsoil, so jet according to altitude and mixture.
3) aftermarket pipe (anything is better than stock)
4) aftermarket silencer/SA
5) aftermarket reeds (Vforce recommended)
6) NGK Iridium IX plug (BR9EIX)
7) install stiffer fork springs (at least .38s from a KLX300) with new 2.5 or 5w oil
8) port the cylinder[/size]
9) have the cylinder head squish redone[/size]

That will get the bike's motor and jetting where it should be. It will be a different bike all together with those mods. Be sure that you inspect the clearances and replace internal parts as needed since these are old and used bikes. Also inspect the power valves, clean them and be sure they are timed properly.

The suspension will still be hurting. I found a set of 43mm forks off a early 90s KX250 and rebuilt them, then installed them on the bike with the KX front wheel. You will lose the odometer but with aftermarket units like the trailtech setups, you can easily upgrade there. The KX front suspension is a must do upgrade for anything faster than 1st gear on these bikes! For the rear, you can simply revalve and respring the stock shock.



Where parts come from:

Keep in mind that the 91-94 KDX250 was built off the 88-91 KX250 bike mainly. There are some differences between the bikes, but if you get stuck trying to find a part, be sure to cross reference those year bikes first. There are also a lot of bearing and clutch parts used of the KX500s of the same years.


-Brake Pads: KX125/250/500 89-93. pads and rotors are the same as the 89-02 KX125/250/500 for the 91-94 kdx250. So you can get the ceramic pads and wave rotors. Brake lines seem to be different though same with the Master cylinders. You can run the KX front and rear SS aftermarket brake lines without issue. A great upgrade is a 02+ CR/CRF honda front master cylinder with lever. The 07+ have the linkage for even better performance.

-Steering head stem bearings list the same for the 80-91 KX250 and KLX250 94-96

-For the Fork seals, the 88/91-95 KX250, KLX250/300 fork seals list the same as the KDX250 units (43mm units). The stock is a 43mm ID x 55mm OD x 9.5mm height. you can run the 10.5/11mm tall seals for beatter sealing.
-Fork gaurds is the same as the KLX250/300. You can run the 90-93 KX units also, but they offer no side protection to the forks

-Rear brake holder/bracket -- same as 89-94 KDX200. Use this for the rear shark fins & scotts gaurd

-Front Forks: Kayaba. .35 kg for 92-94, .34kg for 91. 43mm inverted. (similar forks as the KLX 250/300 bikes). compression only adjustment and very poor forks for anything but slow speed work.

-Rear shock: 4.8kg for 92-94, 5.0kg for the 91. Spring same for 88-02 KX and 89-94 kdx200. Eibach 996 series

-Sprockets seem to be a KX/KDX thing. No differences there.

-aftermarket Chain rollers = Small

-Chain Guides: 88-96 KX 250/500, KDX200

-Reeds are the same as the KX500 units in FMF. Boysen same as 89-96 KX250. Mototassinari use the KX500 Vforce kit.

-Lower shock bearing/seals is the same a 88-01 KX500

-Unitrak bearings are the same as a 88-01 KX500

-Rear swing arm slider is the same as the 94 kdx200 , 89-01 KX500

-Rear mudflap is the same as the kx250 (90-91)

-Looks like the seats are a bit different, but a cover would most likely fit both From a 91-94 250 or 89-01 KX500.

-Clutch: 89-01 KX500, 88-91 KX250

-Clutch cable 90-91 KX250

- Throttle Cable : 88-91 KX250, 88-02 KX500

-Top End gasket, same for 88-91 KX250 (stock thin head gasket .26")

-Piston Needle Bearing 74-02

-Connecting Rod 78-02 KX250

-Piston & rings KX250 90-91

-rear bearings wheel bearings: Timken 104 series bearings. You can pick them up at any autoparts store.

1986-2002 KX125/KX 125/KX250/KX 250 and KX500/KX 500, 1989-2002 KDX200/KDX 200/KDX220/KDX 220; 1991-94 KDX250/KDX 250; 1993-96 KLX650A/KLX650R/KLX650/KLX 650; plus the 1983-84 KL250C/KLR

-Front bearings:

Timken 202 series bearings. You can pick them up at any autoparts store.

KAWASAKI: 1983-83 KX125/KX 125; 1968-70 F3 175; 1983-96 KDX200/KDX 200 1969-70 F4 250; 1971-72 F8 250; 1973-75 F11 250;1982-84 and 1991-94 KDX250/KDX 250; 1994-95 KLX250D/KLX250R/KLX250/KLX 250; 1970-75 F5/F9 350; 1993 KLX650A/KLX650R/KLX650/KLX 650; 1993-96 KLX650C.


For the plastic, you can easily run KX front fenders and number plates. The side plates and radiator scoops are KDX250 specific. I used a 1990 KX250 rear fender for a MX look with minor trimming.
Last edited by argclh6670 on 05:00 pm Mar 09 2013, edited 1 time in total.
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Julien D
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Re: KDX250 info

Post by Julien D »

Stickied, thanks for writing this up!
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KDX250 info

Post by zz3gmc »

The KDX250 chassis wasnt based off of a KX like the WRs and RMXs were. The frame was very differant from any of the KXs of the time. I raced a 90 KX and you couldnt swap plastics, tanks easily like you could with the WRs and RMXs with there MX counterparts (YZs and RMs) I remember seeing pics of Team Green KDXs where they tryed to adapt KX tanks and seats to get the better ergos of the KXs with out much success. The engines definately had KX lineage.
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Re: KDX250 info

Post by argclh6670 »

You are right, the chassis NO. Engine yes (wide ratio trans and heavier fw) and that is what I was referring to.. if the chassis was. Like the kx then the bike would have been much more popular and had a much longer production life.
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KDX250 info

Post by zz3gmc »

I wanted one bad when they came out! Alot of racers bought them and did fairly well with them. My friend bought a new 92 and I tryed it out. I loved the motor, his was uncorked, but couldnt come to terms with the handling in the tight woods. Especially coming off of a KX250. I ended up jumping over to the 200 class and bought a 92 KDX200. The 200 just out handled the 250 in the tight stuff but was down on power compared to the 250. I do have a soft spot for the 250 and wouldnt mind picking one up someday.
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Re: KDX250 info

Post by jgas »

Where I look for info on older mdx 250s? I have an 81 and am buying a 79 for vintage racing in AHRMA and also for trail riding. Thanks.
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Re: KDX250 info

Post by jgas »

Duh, never mind! I just found the air cooled section.
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Re: KDX250 info

Post by Ron-e »

Do you know if the 94 KX 500 rear wheel will fit the 91-94 KDX 250? I know its different tire size but will the whole rear wheel work, spacing, brakes, ect.
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KDX250 info

Post by dingerjunkie »

Just wanted to consolidate information to this thread with a question related to parts compatibility.

Radiator options...one thread stated that the 88-89 KX250 radiators are a direct bolt-on. Has anyone here on the board done this swap to verify before I drop coin on a new set from Myler's? They make a really nice set for the '89 KX250, but I'd hate to waste the money and find out that they are not actually a bolt-on swap for the 91-94 KDX.

Who has replaced successfully with what for radiators on the KDX250D series?

UPDATE: did some digging at BikeBandit. Left-side radiators between '98-KX250 and KDX250D share part numbers. Right side does not. Is anyone familiar with what will fit the right (pipe) side?
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Re: KDX250 info

Post by Chuck78 »

I've got a ton of info compiled from this forum and others, and some which is identical to this 1st post above, for my friend's KDX250 as I'm basically his mechanic.

I'm going to dump all my cut & paste info below, and edit it down as time allows:


These bikes aren't terribly exciting as they're set up factory stock (jetted overly rich, & poor suspension setup), but with carburetor jetting + leaner slide + different jet needle, KX250 thinner head gasket, aftermarket KX250 pipe, VForce3 or VForce4 or Boyesen RAD Valve reed cage, and RaceTech front suspension mods, they are monstrous fire breathing beasts... A fair bit heavier than a KDX200/220, but you won't need any more power than what these can be tuned for, that's for sure... Their weight makes them feel like a 4-stroke in the tight woods riding terrain, but their power and wheelbase makes them incredible hill climbers and awesome desert bikes.


_____________________________________________________

KDX250 Carb stock specs(from parts fiches):
Slide #5
(***go leaner to #6 regardless)

Needle R1368H stock (too rich)
R1366H opt. (Too rich even in 2nd notch from top w/#6 slide)
***R1368H opt.
R1369H opt.
R1370H opt.

Main jet #165 stock
160, 162, 168, 170 opt.
(**155-158 w/40:1 & mods)

Pilot jet #42 stock
38, 40, 45, 48 opt.
(**38 w/50:1 & exhaust/kx head gask. mods)


*Top End gasket for 88-91 KX250 (head gasket thinner .026" vs .046" stock KDX, helps boost compression)
*VForce3 reeds
*try putting the stock needle in the second notch from the top (carb will still be jetted too rich without swapping the needle & slide)
*a #6 carb slide with 1mm more cutaway than stock will help (or grind approx. 1mm off the bottom of stock slide)
*Remove the airbox snorkel from the lid and cut the lid open to maximize air volume intake.

------------------------------------------------------
Carb - #6 slide, 1367 needle, and 38/155-158 jetting.

------------------------------------------------------

Rejet the carb -- 38 pilot jet, 158 main, #6 slide, R1367 needle 3rd clip. (Above is for 50:1 Amsoil premix)

Mod a 90-94 kdx200 fmf silencer
OR
DEP silencer & DEP expansion chamber for 89-93 KX250

-----------------------

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

You need a 1988 KX250 head gasket , .026" thick
Mod a 90-94 kdx200 fmf silencer
+ new KX250 pipe
VForce3 reeds
Carb - #6 slide, 1367 needle, and 38pilot/155-158 main jetting.

Once you do that, the motor RIPS. Especially with a port job.
-------------
DEP rear silencer and DEP front pipe. Which incidently is a KX250 89-93 version.
-------------

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 with RaceTech Gold Valves and put stiffer springs in; the stock forks have way too much high speed damping and not enough low speed damping, and way too soft of springs. With the stiffer springs (approx 0.40kg/mm) run virtually 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 position.

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

Shorten the chain and run the wheel all the way forward, the swingarm is quite long (MX length) for woods riding.


-------





SLIDE SWAP:
Jetting, you need that leaner #6 slide. Also check eBay for a early 90's KX250 carb and it will have what you want. I know when I had the bike, I looked at the KX250 jetting specs and started there. took about 4 rides to get it completely dialed in and those were my settings. Altitude and temp may have slide adjustments on the pilot/main, but that should be your ticket.


PIPE:
I'm also keen on putting a Pro Circuit pipe and silencer on it and im pretty sure (if I've absorbed all the above information correctly) that part # PK89250P is the correct pipe and the correct silencer is part # SK89200D-304 can somebody please confirm this for me please?




I believe everything below is an identical copy of the first post above, I'll check later, just a quick info dump right now:

https://www.thumpertalk.com/forums/topi ... -us-bikes/
By JJrace


----------------------------------
The KDX250 is a lot like the Yamaha WR250 and Suzuki RMX250 of similar years. They are an enduro model based off of an MX bike from the factory. They suffer from poor jetting, engine performance and suspension.

The factory added an XR style rear chain adjuster, kick stand, larger tank, wide ratio tranny, different cylinder porting, a heavier flywheel rotor, and lighting stator to the KX motor make up the mods to make it a KDX model.
To get the bike into peak performance, you will have to start with the motor, then jetting, then suspension.

Where to start your modifications:

]freebies:

1) Remove the airbox snorkle from the lid and cut the lid open to maximize air volume intake.

2) put the needle in the second notch from the top (carb will still be jetted to rich)

3) set the sag on the rear suspension

now you will find that the bike still has carb issues, performance issues and suspension issues. So you will be putting some money into the bike to make it into what I can become.

]money mods:

1) KX250 thin head gasket from 1988 (.26 vs .46 in thickness)

2) Rejet the carb -- 38 slow, 158 main, #6 slide, R1367 needle 3rd clip. This will bring the bike closer to the KX spec.

** I ran 50:1 amsoil, so jet according to altitude and mixture.

3) aftermarket pipe (anything is better than the poor stocker)

4) aftermarket silencer/spark arrestor

5) aftermarket reeds (VForce3/4 recommended)

6) NGK Iridium IX plug (BR9EIX)

7) install stiffer fork springs (at least .38s from a stock KLX300) with new 2.5 or 5w oil

8) send out the cylinder to be professionally ported

9) have the cylinder head squish redone (Jeff Fredette or Tom Morgan Racing)

That will get the bike's motor and jetting where it should be. It will be a different bike altogether with those mods. Be sure that you inspect the clearances and replace internal parts as needed since these are pretty old and well used/abused bikes. Also inspect the power valves, clean them and be sure they are timed properly.

The suspension will still be hurting. I found a set of 43mm forks off a early 90s KX250 (91-95 KX125/250) and rebuilt them, then installed them into the KDX250 triples with the KX front wheel/axle. You will lose the odometer but with aftermarket units like the trailtech setups, you can easily upgrade there. The KX front suspension is a must do upgrade for anything faster than 1st gear on these bikes! For the rear, you can simply revalve and respring the stock shock as needed.


Where parts come from:

Keep in mind that the 91-94 KDX250 was built off the 88-91 KX250 bike mainly. There are some differences between the bikes, but if you get stuck trying to find a part, be sure to cross reference those year bikes first. There are also a lot of bearing and clutch parts used of the KX500s of the same years.

-Brake Pads: KX125/250/500 89-93 pads, and rotors are the same as the 89-02 KX125/250/500 for the 91-94 kdx250. So you can get the ceramic pads and wave rotors. Brake lines seem to be different though same with the Master cylinders. You can run the KX front and rear stainless steel aftermarket brake lines without issue. A great upgrade is a 02+ CR/CRF honda front master cylinder with lever (same as '95-'06 KDX). The 07+ CRF have the linkage for even better performance.

-Steering head stem bearings list the same for the 80-91 KX250 and KLX250 94-96

-For the Fork seals, the 88/91-95 KX250, KLX250/300 fork seals list the same as the KDX250 units (43mm units). The stock is a 43mm ID x 55mm OD x 9.5mm height. you can run the 10.5/11mm tall seals for better sealing.

-Fork guards are the same as the KLX250/300. You can run the 90-93 KX units also, but they offer no side protection to the forks for woods riding and rocks.

-Rear brake holder/bracket -- same as 89-94 KDX200. Use this for the rear shark fins & Scott's rotor guard

-Front Forks: Kayaba. .35 kg for 92-94, .34kg for 91. 43mm inverted. (similar forks as the KLX 250/300 bikes). compression-only adjustment and very poor forks for anything but slow speed work. RaceTech Gold Valves and .38-.40kg springs or swap 91-95 KX125 forks/axle/wheel.

-Rear shock: 4.8kg for 92-94, 5.0kg for the 91. Spring same for 88-02 KX and 89-94 kdx200. Eibach 996 series

-Sprockets seem to be a KX/KDX thing. No differences there.

-aftermarket Chain rollers = Small

-Chain Guides: 88-96 KX 250/500, KDX200

-Reeds are the same as the KX500 units in FMF. Boyesen same as 89-96 KX250. Moto Tassinari use the KX500 Vforce kit.

-Lower shock bearing/seals is the same a 88-01 KX500

-Unitrak shock linkage bearings are the same as a 88-01 KX500

-Rear swing arm slider is the same as the 94 kdx200 , 89-01 KX500

-Rear mudflap is the same as the kx250 (90-91)

-Looks like the seats are a bit different, but a cover would most likely fit both From a 91-94 250 or 89-04 KX500.

-Clutch: 89-04 KX500, 88-91 KX250

-Clutch cable 90-91 KX250

- Throttle Cable : 88-91 KX250, 88-02 KX500

-Top End gasket, same for 88-91 KX250 (stock thin head gasket .026")

-Piston wrist pin Needle Bearing 74-02

-Connecting Rod 78-02 KX250

-Piston & rings KX250 90-91

-rear bearings wheel bearings: Timken 104 series bearings. You can pick them up at any autoparts store.

1986-2002 KX125/KX 125/KX250/KX 250 and KX500/KX 500, 1989-2002 KDX200/KDX 200/KDX220/KDX 220; 1991-94 KDX250/KDX 250; 1993-96 KLX650A/KLX650R/KLX650/KLX 650; plus the 1983-84 KL250C/KLR

-Front bearings:

Timken 202 series bearings. You can pick them up at any autoparts store.

KAWASAKI: 1983-83 KX125/KX 125; 1968-70 F3 175; 1983-96 KDX200/KDX 200 1969-70 F4 250; 1971-72 F8 250; 1973-75 F11 250;1982-84 and 1991-94 KDX250/KDX 250; 1994-95 KLX250D/KLX250R/KLX250/KLX 250; 1970-75 F5/F9 350; 1993 KLX650A/KLX650R/KLX650/KLX 650; 1993-96 KLX650C.

For the plastic, you can easily run KX front fenders and number plates. The side plates and radiator scoops are KDX250 specific. I used a 1990 KX250 rear fender for a MX look with minor trimming.

------------------------------------
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
'62 GMC 1000 Panel Truck
'88 Suzuki Samurai TDI/Toyota swaps
'88 Toyota 4x4 pickup
jedidan45
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Re: KDX250 info

Post by jedidan45 »

Thanks SO much for your additional info Chuck78!!
Long live the KDX and all hail King Fredette!
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