240 Big-Bore Kit for the KDX200

Problems
Complaints about the KDX 240 kits are that they will not rev out as much, that they will not idle, that they use too much fuel, that they run hot and tend to seize.

My Thoughts
It seems to me that several of these problems could be at least partially attributable to incorrect jetting. If my bike is typical, it shows that the 240 jetting requires considerable leaning at the bottom and richening at the top. My bike would not idle reliably nor pull cleanly off idle, but several things I did helped: I reduced the pilot jet to a number 40 size, I ground away the slide cutaway by at least 1.5 mm, and I thickened up the straight section of the needle temporarily with silver bearing solder until I either find the right needle or replace the carby with a bigger one.
The needle does indeed affect the just-off-idle and even, to a small extent, the idle mix itself. If it's way rich, as mine was, no amount of fiddling will yield a setting that will allow the bike to both idle steadily and roll-on cleanly.
With these mods (and my small 28mm carby), the fuel economy is amazingly good (better than my mate's 407 four-stroke), with responsiveness to match. It now idles semi-faithfully (but not perfectly) and rolls on crisply. The low- and mid-range grunt is superb, producing controllable power that is ideal for tight trail-riding. It has a distinct powerband, but it doesn't hit savagely. Even with the tallish gearing, wider-ratio SR box, heavier flywheel, and small carby, it will still lift the front wheel in the first four gears.
I don't know how important grinding the slide cutaway is as I ground mine at the same time as performing the other mods. You might be able to produce satisfactory results without this step, though it costs nothing and is worthwhile in my opinion.
I am also running a 180 main jet, which came with the bike. It seems only slightly rich, but a more strongly-tapered needle is what the bike really wants, with a somewhat smaller main jet (my 1/2 throttle range seems to be slightly on the lean side).
I imagine that many 240-kitted bikes have been run with near-standard jetting that worked well in their previous 200cc incarnation, and have suffered from not only poor economy and richness at the low end, but also poor power and overheating caused by leanness at the top.
This is not to say that the thin iron bore does not run hotter by nature. Of course it does. Ask for a good 3 thou' clearance if you have the job done, and check your ring gap carefully. I seized mine because of a lapse of concentration when I held it wide open only with only a few hours on the rebore. Wiseco pistons require a longer run-in time because they are forged and expand more than the stock cast pistons. I also recommend using more oil during run-in with these sleeves - say 25:1 for a few tanks.
As for the lack of top end zap, this is to be expected with any big bore kit especially if the ports are not reworked to compensate. See "Port-Time Area" on this page. The head should really be re-machined too. Also, high-end breathing is affected by carby size, already a limiting factor with the stock 200 let alone a 240. Having the carby bored out to 36mm, as done by RB Designs in the US, would be very beneficial to the 240 I would think. Until I buy a bigger carby, I can't really comment on this issue as mine will inevitably sign off early. Even so, it pulls so hard and furiously before the power quits that I can't help wondering if the right jetting in a good 36mm PWK wouldn't almost give it a top end to match.
I recommend running Shell Optimax with the 240 kit. As I write I've just switched over from BP premium, and the difference is quite noticeable. I had to screw my throttle stop in a couple of turns as soon as I fired it up on the new fuel to prevent it revving so hard. It idles better, pulls better, pings less (my cylinder head hasn't been machined), and seems to be more economical too. Its (presumably) faster burn speed is ideal for the wide bore and extended combustion chamber formed by the 240 kit. Note that if you're already running rich, Optimax will make the richness worse. I switched over in mid-winter when the air is denser, so some of the power improvement I felt would be simply a result of Optimax effectively correcting for this.
While on the subject of economy, my bike, with its 28mm carby, always was excellent and is now probably even better on Optimax. For example, my last ride was 90km (56 miles) of quite fast, steep, tight and often very rough terrain, with no attempt to spare the horses, and I used only 7 litres (1.85 US galls) of fuel. Not bad.

These kits use an LA Sleeve liner and Wiseco piston. You might not see the pistons listed in the Wiseco catalogue, but performance shops should be able to source them. In Australia, Serco handle the 240 kits. Eric Gorr says that FMF's Fatty pipe is ideal for a bored engine (I have one). You need to have your head machined to suit, otherwise there's not much hope of good idle and off-idle permformance as the squish band will be wrong.

See this thread for a further discussion of the 240 kit. Search on "240" for more.

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My 240cc big bore sleeve as seen from below (bored to first oversize)