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I bought my first KDX today, an '03 220. The previous owner swapped out KX250 forks and KLX 300 Swingarm. The bike seems to have low hours and a top end rebuild with stock piston was just completed. The bike starts fine and runs strong on the bottom end but falls flat in the mid range and will not rev higher, feels like there is something clogged in the carb to me. Does this sound like a carb issue? I am thinking about getting the RB mod on the carb and head. Any other common issues I need to be aware of or items that should be checked/replaced? Also, how can you tell a 200 from a 220 by looking at the bike, seller said it was a 220 but I am not 100% sure.

Thanks for the help!

Look at the right side of the cylinder, should say 216 or 198 (220 or 200 cc, respectively).

I know you don't want to hear this, but you need to replace the stock piston. I just bought an 04. Started + ran great. I rode it exactly once. Got my replacement piston (Wiseco!!) and swapped it out to find that the stock piston was broken. Half of the skirt in the back had departed from the piston and went into the crank. It still ran like that, but it could have ruined the bottom end, or even the cases. So please take my advice and EVERYONES and replace that piston!

Second: The 220 really does not have strong mid/top range power. While it is impossible to know what you are experiencing, it is most likely HOW the 220 feels stock. You should send your head + carb off to ron black (RB-designs) while doing your piston.

All together you are looking at about 400 bucks total (piston, carb, head) and your bike will be a rocket. You can also put some new reeds in for CHEAP. These will deff. help. Get a set of Boysen carbons for like 40 bucks. Then save the rest of your money on gas, tires, chains because your KDX will now last pretty much forever.

~Ian

OK! Thanks for the great info. Yes, it is a 220 as the seller indicated. The piston is supposed to be new but stock size. I am not sure if they went OEM or wiseco. Can I tell by pulling the head? The issue with the mid range is definitely not just power band. It is most likely a Carb isuue. It falls flat at about 3k and will not even go higher.

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falling flat at 3 K is definitely not healthy
go through the carb, clean her up and jet her properly

maybe check the reeds aswell

I think you might be able to tell by the markings on the top if the piston if it is a wiseco or not
might even be a better idea to loosen the cylinder from the bottom and lift it off, leaving the head on
that way you can see the entire piston and you only need to replace the base gasket,or if your lucky not even that

TNKDX,

didn't know you meant 3K rpm falling flat. That DEFF sounds wrong. Have RB go through it.

You can tell if you pull the head. My OEM only had an arrow (pointing forward) and a letter "A" written on the top of the piston. The wiseco has an arrow and a part or serial number. I have both so I can probably get a picture for you this week.

Awesome!! Thanks so much. Also sounds like new reeds would be a good idea. Any other basic stuff I should check while I am in refurb mode?

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3K is hardly above idle. "Midrange" when the power valve is opening is around 6K.

A stock 220 to me feels like something is wrong with it. They don't rev. They probably make peak power around 7200-7500 rpm and sign off almost instantly.

That said, if the top end was just redone I'd suspect the power valve was not assembled properly and therefor not working right. That'll kill an hopes of upper end power.

Thanks for the help. Since it does nor have a tach the rpm is a guess. You might be right could be closer to maybe 5k. When it hit this mark it is like hitting a wall. Like it is starving for fuel. It bogs and will not pickup rpm's. I am pretty sure this is before 6k but could be wrong. Supposedly the engine was rebuilt including new rod and crank by the local dealership. So I assume the powervalve is working but maybe not. I saw a YouTube video that showed you could check the operation by removing the inspection cover and taking the engine past 6k.

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OK. Talked to the original owner and when they did the rebuild they used the stock piston. So looks like I will get a wiseco to go with the RB head and Carb mod. Should be good to go then.

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disassemble + clean the KIPS when you do your topend. Clean it good!