1991 KDX 250. Where can I get Power!!!


May 30, 2001
I recently bought a mint 1991 KDX 250. The issue I am having is that I'm finding that it lacks in the power department. It is totally stock ecept for a ProCircuit silencer. I even have trouble getting the wheel up in second or third gear over a puddle. Can anyone offer any tips (through experience is possible), what I can do to get more power?? (pipes, carb set up/jetting, etc). ANy assistance woould be appreciated. The more detailed the better so I can go shopping and get the KDX going! Thanks in advance.:)


Mar 23, 2001
I think you answered your own question. A pipe plus the proper jetting should give you ample power providing your piston and ring are in good condition. If you need MORE low end for lifting tire, etc you could try going a tooth or two larger on the rear sprocket. Lose a little top speed to have more wheelie poppin power.


dirt bike dave

Sponsoring Member
May 3, 2000
The engine is basically a mildly ported '88-90 KX250 with a very heavy flywheel; there is power in there with some mods. Before you sink moneyh into mods, make sure the rings and piston are in good shape and the silencer a nd KIPS valves aren't clogged as a result of the stock rich jetting.

The stock jetting is way too rich and it is common to modify the slide or replace it with a KX unit with 1mm more cutaway for better low end response. My buddy and I both had KDX250's, can't remember the exacat jetting we used but I think it was a 160 main jet, 38 pilot and one or two steps leaner (higher #) on the needle. Also, for trail riding, you will probably be happier with a B8ES or even a B7ES plug in place of the too cold B9ES.

With the VERY heavy stock pipe and silencer but revised jetting, the bike has a strong low/mid hit but does not rev out very well. An FMF pipe will smooth the low end hit out and get you some revs on top, but it is a good low rpm trail motor first and foremost.

Use a thinner head gasket from a '88-90 KX250 for more compression and more power. It will run fine on pump premium.

Use Boyesen reeds. I had a RAD valve too, worked OK. Got it cheap, but it is probably not worth the retail price.

On mine and my buddies, there was tons of casting flash in the intake port - clean it off with a dremel tool next time you do a top end. Also, on both of ours, the openings of the side KIPS valves stick up well above the floor of the main exhaust port when the KIPS are fully open. I used a porting tool to remove some material so that there was a very smooth transition from the KIPS valve to the main exhaust port. This had no negative effects but really increased revs and peak power.

Oh yeah, a 13t countershaft sprocket is good for tight trails and will certainly help the bike feel more peppy.

Best of luck with yours - it can be a decent running bike with some minor mods. If you want major power, you are going to have to port it, but with minor work it's a nice motor with strong low end for a 250 2 stroke.

Also, revalve and respring the forks! The stock fork settings are pretty grim in the rocks and roots. BTW, it is a hard bike to ride wheelies on as the wheelbase is pretty long and the steering geometry is slower than most trail and MX bikes.
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Mod Ban
Dec 10, 2000
Everything dave said is spot on, even the jetting is close to the same as what I am running. One additional thing. That engine has a tremendous amount of untapped power inside it, all it needs is a little grinding from someone that knows what they are doing. My bike will loft the front wheel at any time in the first three gears, and I can get it up in fourth with a little coaxing. My bike is the king of low-end grunt for hill climbing, and will hold it's own against much newer bikes in acceleration, as well. The only thing I am lacking is top-end overrev, but that doesn't matter in tight woods.
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