kdx 250 engine swap with kx 250... Is it possible?

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#1
Hi, a freind of mine owns a 91 kdx250. The other day, he asked me if it was possible to swap out his old cylinder for a kx cylinder of a similar yearr. Not knowing the answer, I caame to you guys for help. I was thinking it could be done, but I don't know wich years interchange. I was also thinking the carb might not be the same modle so therefore, a kx carb should be used as well as a kx ignition coil. Again I'm not sure of this, thats why I wanted to check with you guys.

Thanks

Eric
 
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#3
Ok, so he'd have to use a kx head too? anyways, I think that's what he intended to do. Making up some brackets should not bbe too difficult.
Thanks.

Eric
 
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#6
Why would you want to Change a KDX?
I mean, dont get me wrong, but when you buy a KDX, you should like it that way, or trade it in for a KX and put Enduro Parts on it...
 
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#7
Back to your original question: can you swap a KX cylinder into a KDx250?

Yes, 88-91.

Heads are different.

Cylinders are the same. After 91, the base design changes and KIPS and entire motor.

Make sure you know what you are asking.
 
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#8
My freind wanted to change it cause his kdx cylinder needs replating and he has a good deal lined up on a kx engine.(it would be cheaper to get this engine than to have the other one replated. he also likes the top end power of the mx bikes compared to the kdx's low end grunt(he'd like to have more power on top but still have good lowend power also. I figured that with the heavier flywheel on the kdx and longer gearing and the power characteristics of the kx, the bike would stay managable in tight trails, but would revout more. But correct me if I'm wrong as I know very little about kdx's and I wouldn't want to steer him in the wrong direction.

Thanks

Eric
 
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#9
Now at least i know why your friend wants to do it...Money is almost everything in dirtbiking, But I dont see how It would work...
 
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#10
Well the KDX motor is more than just what you think different.

The flywheel is not that much different in weight, it is the extra drag from the lighting coil.

Stock you get thicker head gaskets on the KDX, the WR gears, different water routing thus different heads, lighting coil, different carb setup and pipe.

Really if you install the KX .26 headgasket, a good pipe, freshen the reeds (the VforceII is the best way) and jet the carb up leaner with a 1367 needle, 38/40 pilot, #6 slide, 155-160 main you will have a 88-91 KX power with the WR gears and lighting.

now chassis and suspension is another story.

For the cylinder repair, talk to Eric Gorr. You can sleeve the unit and run a 265 kit in there. Also get the cylinder ported out while it is there.
 
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#12
Thanks for the help matt. I was under the impression that the 2 cylinders porting was completely diffferent and by changing to the kx cylinder, the power delivery would be different. As I said, I dont know much about KDX's, thats why I asked for help :) thanx. As for the lighting coil having a drag effect on the flywheel, I have to admit, I didn't even think of that. The thicker head gasket will lower comp, increasing top end, but will the low end loose alot of power or will it stay reasonable? Also, did the kdx and kx use the same carb? And what mods do you recommend to improve handling?

Thanks again

Eric
 
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#13
More compression = more power everywhere.

Running the thinner head gasket will make a noticeable difference in the power of the bike. Especially with a pipe and proper jetting.

Be sure to remove the air snorkle from the airbox lid also. Be sure to have a clean filter and fresh packing in the silencer. Start with a new plug. I run the Iridium NGKs the BR9EIX units. They give the most energy to the spark of any of the NGK plugs. In simple terms, gives you more power in the 7/8-full throttle range.

The carb is the same PWK38 unit on both bikes. The KDX is jetted like a fat pig stock.

Here is what you need to get close:

Air screw (2 turns out from full)
1367H needle (next leaner offered from Kawasaki)
#6 slide (stock is #5, the 6 is leaner)
38 pilot
155-158 main depending on conditions/altitude.

It will cost you about $100 to get the new needle and slide along with some brass to tune for where you are. Well worth it on the bike!

Like said a pipe (I run the Dynoport unit. Gives lots of power and is much thicker than the PC and FMF units) and V force II reeds will help the motor along with the thinner head gasket from the KX.

Suspension, wow! First off lighten the bike. Remove the extensions off the subframe that hold the fender up. Toss the rear fender and get a 91 KX250 unit.
The front forks are tough to get working properly. But run a .40 spring +/- .01 depending on your weight ( I am about 170 usually!). Run a light 5 weight oil 4.5" from the top. You will also need some SealSavers to help keep dirt from the seals. Worth the $20 there. Raise the forks about 5-7mm in the clamps. I ran the compression like 10 clicks in and that worked well until I got into a race speed of pace. At that pace, the lack of rebound really effected the bikes handling in the rough stuff. So now i have some KX forks from the same year on the bike.

For the bars, replace the rubber cones with aluminum units. About $30 there. I personally run the ProTaper woods bars and really like them. They are worth the money!

For the rear, play with the clickers until you get them right. Run about 100-105mm of sag back there. The stock spring is about right for most people. I got a deal on an Ohlins stock and installed that. Seems more controlled and plush than the stocker, but the stocker needed new fluid and a recharge to give it a fair shake.

Beyond that, I have cut down the seat foam to get lower on the bike. Others have ground down the steering stops to allow more turn there. I have not found the need yet to do that.

Hope that helps. I do have some links on setting up the clickers and sag on the bike in general if you need them.