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I'm finally getting around to jetting. Here are the specs:

'06 200
160 Main
1174 needle, 3rd clip
48 pilot
5 slide
Woods Gnarly, no dents
Turbinecore 2
Recent top end with great compression
Elevation: 100-200 above sea level to 600-700 above

Doing the pilot first, I went all the way down to a 40. This seems extreme, so I wanted to make sure I'm doing it right. I screwed the idle in for a little gas, then adjusted the air screw until it got even faster. With the 45 pilot, it took almost 5 turns out to get to the fastest, 42 was around 4, and 40 was 3 turns.

Logic tells me a 38 or even 35 is what I need--that seems way too small! Could I be turning the idle too high or low during this test? What RPM should it normally be idling at?

I'm currently running a 38 in mine, and 145 main. 140 psi compression, gnarly pipe,boyessen reeds, fmf silencer, twin air filter and airborne lid cut 1/2 way. Can still go leaner on the main. 88 degrees, 90% humidity, 25 ft above sea level

Mayor Brap Brap wrote: Doing the pilot first, I went all the way down to a 40. This seems extreme, so I wanted to make sure I'm doing it right. I screwed the idle in for a little gas, then adjusted the air screw until it got even faster. With the 45 pilot, it took almost 5 turns out to get to the fastest, 42 was around 4, and 40 was 3 turns.

Logic tells me a 38 or even 35 is what I need--that seems way too small! Could I be turning the idle too high or low during this test? What RPM should it normally be idling at?

That was a lot of work figuring that out, going through all those jets! According to all the jetting guides I've read, that's how it's done. I don't think the beginning idle speed has to be at any specific RPM, any reasonable RPM will do to use as a baseline (just like you did). That is, according to the jetting guides I've read.
35-38 does seem kind of small, I have to agree. Your at a pretty low elevation where you would expect to be able to pump more fuel with the increased oxygen you get.
Let us know what you decide on, i'm curious now!

From what I've read, these bikes were super rich stock--maybe so they wouldn't explode easily? I've also seen a lot of guys in here run 38 pilots, which seems to be the sweet spot. I guess a modified question is would it be okay to be slightly rich on the pilot? If I'm at 40 and it takes more than 2.5 turns out for a faster idle, will I really notice the difference during riding? Also, the bike ran well with the big jets, so that further complicates everything.

Is the E-series the same as the H-series as far as jetting goes? My E-series KDX200 owners manual lists the acceptable jet sizes as: slow 42-52 (48 stock) main 145-170 (158 stock). My bike is running fantastic on 45/155 jets. Never fouled a plug, minimal spooge from exhaust no dripping or anything. I have not experimented with much smaller jets.
When you experimented with the smaller jets, did you notice an improvement?

If you're pilot is too lean, you'll feel it. It will be a bit of a dog down low when you're under load.
It may be that your big main jet is over richening the whole circuit. Although not technically right, I'd first install say a 155 main, and then recheck your pilot circuit. Or to be technically right, go through the whole jetting process, and then go back to your pilot circuit one more time.
Every bike is different, but just for comparison sake I'm running a 40 pilot in my 200, at about 1000 ft. But I've got the much richer DEK needle installed, which probably would equate to about a 45 with your needle.

Seems like good advice from Calkawi ^ about the jetting starting points. Here's a link that may provide some more info. http://www.kdxrider.net/forums/viewtopic.php?t=4140 Post #7 by Morie will be of use to you, he describes an h-series jet needle. It's all a good read.

I knew jetting this thing would be a conundrum! Every other bike I've had––ZRX1200, CRF230––I've been able to jet based on recommendations of different jet kits/professional's experience. All I had to do was provide my location and riding style, order the jets, and it was plug-and-play success all around.

With the KDX, it seems like a crap shoot. On one hand, you have Fredette saying that stock jetting is fine with a full FMF system. Then I see guys here saying how the KDX is way too rich and going down to way smaller jets (I see the 38/155 combo on here a lot).

I never thought the Main had any effect on the whole circuit––isn't that just for top RPM range? If I do go down in the main, should I keep the 40 pilot in, or go back to the stock 48?

All I know at the moment is the pilot circuit is asking for more air. Maybe I'll keep the 40 pilot and go down in the main to see if that makes a difference.

Recommendations will differ. But each bike is different, and riding styles are different. If you're really "on it" when you ride, then the big stock jets might be just fine.
I wouldn't worry about all the bikes on here with 38 pilots. I think most of them are compensating for a richer needle choice.
BTW, I really started to appreciate my KDX when I started understanding how to dial it in for my riding style. I think it's well worth the effort taken.

Just be careful Mayor as your bike is plated it is safer to be a little too rich on the pilot.

I just put the stock jets back in and lowered the needle a clip. Honestly, it runs fine this way. While the jetting guide maybe help a lot with fuel efficiency, I really was put off with how lean I was going.

The jetting guide is for performance. If you want to stick with a 40/42, that will be just fine. The "Fastest idle" method is just to get you in the right size, you still need to adjust the airscrew for fastest throttle response from closed (under load), it's not meant to be left at the fastest idle. FWIW I run a 38 pilot in my e series.

Mayor Brap Brap wrote:I just put the stock jets back in and lowered the needle a clip. Honestly, it runs fine this way. While the jetting guide maybe help a lot with fuel efficiency, I really was put off with how lean I was going.

I run stock as well in the 220. 145m 42p. May not make the extra hp ,but its better then sticking a piston.

The stock jets will run ok, and are actually pretty close for winter riding. My 200 before jetting wasn't that quick getting on the pipe in summer weather, and another 200 that I helped sort out would struggle even more unless he was really on it. There's a nice little tool you can get at Jetsrus, that makes changing your main jet a breeze, I think it costs maybe 5 dollars? It takes less than 5 minutes to change, and then dial your air screw as you ride. I have to do this maybe 4 times a year to keep my bike running optimally, but the process is little more than putting my boots and helmet on, so why not? My spark plug stays clean, and more importantly my power valves and pipe are probably staying cleaner as well.