KDXRIDER.NET

Hi everybody!

My 2003 KDX220 just quit making power, and I haven’t figured out why. I am posting to ask for direction on where to look next. I do not think it has anything to do with the carburetor.

Scenario: Months ago I was enjoying riding up and down the mountains at Doe, and about 30 minutes into the day, during one of the easy climbs, the engine lost its gusto. It still ran well enough for me to ride it back to the van, but it didn’t have any spunk at all. Power was linear, yet anemic.

Research: When I got home I took the carburetor off and found a speck of trash in the bottom of the bowl. I suspected junk in the tank and cleaned it and the carb thoroughly. That had no affect. I took the carb apart a few more times, playing with jets, adjusting float level, and even tightening down a loose jet block (gaskets were fine). Still nada.
The Spark plug was oily and grungy, so I replaced it. No change. I then confirmed the plug was sparking by laying the plug on the engine and pushing the kick starter. I saw a blue spark.

Eureka moment: When I finally had the notion to put this 2003 carb on my other KDX220 (a 2002 model), that 2002 KDX ran great and was screaming with power. The inverse of that -putting the 2002 carb on the 2003 bike- did not improve anything on the 2003 bike. All that to say that I don’t think the carburetor is the problem.

More discovery: I took the reeds off and they looked fine. Looking through the intake I see nothing concerning. Listening through the intake, I can hear suction as I move the piston with the kickstarter. There is a nickel-sized drop of oil under the bike. It seems to be coming from near the countershaft sprocket. It has not leaked oil in the past. I removed the left side engine cover and it was dry back there.

Note: I rebuilt the top end ~3 years ago. There hasn’t been too much time on it since; I have probably ridden it about one weekend a month.

Any guidance will be greatly appreciated.

If compression is within spec I would start looking at the ignition. Yes, you can still get a spark even with a bad ignition. Since you have another KDX to swap parts from, I would start with the coil/plug wire, then the CDI, and last, the stator.

Compression test would be first on my list.

Thanks for the quick replies. A compression test sounds like a good next step. I'll borrow my buddy's tester this weekend. Is there a tutorial on how to do this (properly) on a 2-stroke?

Open the throttle & kick until the needle stops moving. Tell us what you get.

I was hoping it was that simple. The reeds are off / intake open; should they be reattached?

Try it like it is and tell us what you get.

I stopped kicking when it got to about 90 psi on the gauge. I think that was telling me that I have sufficient compression, correct?

90 psi is on the lower side.

shunpiker: Molly's is correct. 90 psi compression is relatively low.

You guys are great... thank you for the quick feedback. I did stop kicking, but it may have taken more. I'll re-test and report back.

You probably need to start saving for a top end kit.

Is 140 psi closer to what I should expect? Just a tick over the 140 mark was as high as I could achieve.

shunpiker: 120 to 180 psi is two-stroke range. High performance engines should be near the upper end of the range.

Woah... been a while.

I was discouraged over the winter, but I got more serious about things as the days started getting longer. I swapped the electronics on both bikes, putting the electronics from bike a onto bike b and putting bike b's electronics on bike a. I learned a lot. I started with the coils, then the cdi and regulators, and lastly the stators. I rode both bikes at each swap.

The sick bike never ran any better by installing the healthy bike's electrics. And the healthy bike still ran strong with the sick bike's electrics. I never swapped them back.

What the problem was... the silencer/spark arrestor had become clogged.

A new turbinecore, and I'm back in the power. A belated "thank you" to everyone. I hope you have a great summer.

Glad you found the issue.

Cool that you got it figured out! Enjoy the riding instead of wrenching. :-)

shunpiker wrote: 08:20 pm Apr 13 2025 Woah... been a while.

I was discouraged over the winter, but I got more serious about things as the days started getting longer. I swapped the electronics on both bikes, putting the electronics from bike a onto bike b and putting bike b's electronics on bike a. I learned a lot. I started with the coils, then the cdi and regulators, and lastly the stators. I rode both bikes at each swap.

The sick bike never ran any better by installing the healthy bike's electrics. And the healthy bike still ran strong with the sick bike's electrics. I never swapped them back.

What the problem was... the silencer/spark arrestor had become clogged.

A new turbinecore, and I'm back in the power. A belated "thank you" to everyone. I hope you have a great summer.

Very interested in this, did you take any photos? I've been suffering a lack of top end power in my kdx200, I've opened up my stock muffler for decarbonising, if it is the cause this is a pretty significant discovery for kdx owners using the stock muffler.
If and when I will do a write up on opening up the stock muffler

Thanks for the future article!

The stock muffler also uses the turbinecore design, I thought it was just an fmf thing