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kx250 jetting for a 36mm carb

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just got a new 36mm power jet keihn carb to replace the 38 trying to get a bit more torque how do i jet it will it work with my jeys from the 38 or will it be to rich

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 Old post for sure, but curiosity had gotten the best of me. My 03 kx 250 is currently at the shop getting jetted with the 36mm carb. I am hoping to have a kdx feeling motor with a little more. Will post results as soon as the weather breaks and he gets it fine tuned

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 Old post for sure, but curiosity had gotten the best of me. My 03 kx 250 is currently at the shop getting jetted with the 36mm carb. I am hoping to have a kdx feeling motor with a little more. Will post results as soon as the weather breaks and he gets it fine tuned

OK cool - thread may have results after all. :busted:

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You're going to need porting and head work for that kind of motor.  The 38mm is not at all oversized for a 250 2T.

 

You could try making a super thin base gasket.  Lower the ports, raise the compression.  Depending on the squish clearance you may need a thicker head gasket but if not even better.  That'll get you more torque.

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already had cometic build me a 0.10 thinner base gasket and a 0.10 thicker head gasket to lower ports and keep pump gas compression. I am a shorter light weight rider. When the bike comes on the pipe 90% of the time I lose traction. I was racing a kx/kdx hybrid before I went to the 250. I absolutes loved the kdx power curve but couldn't hang with the 300's in the open fields. The 250 is trickier for me in the woods as I have to stay just under the bike really coming to life. A 47 rear sprocket really helped this but I wont rest till I see what the 36 does. It may be a brilliant move or a waist of time. But I will be the guinea pig on this one!!!!! 

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I am going to run a 36 PWK on my 03 250 woods bike, starting with 40-42P, 165M, N8RG needle #3, 7 slide.

But its too dam cold in the northeast to even test ride it.

It should add bottom end along with a Gnarly and having the head cut for a tighter squish.

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Another thing that comes to mind would be swapping out the reed cage stuffer and carb boot for one from a 99 KX250.  The 99 boot has a narrower cross section between the reeds and carb, which means less volume and higher velocity for any given amount of air flow.  I'd expect better low-mid and improved throttle response.  There may be other years that came with the same narrow cross section intake parts but I know for sure the 99 uses them.

 

 

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Good idea about the reed cage. I just finished going around with boyesen about a similar thing. The reeds they sent me for my 03 were to small and wouldn't seal on the corners. After two phone calls I sent them back. They are sending me some for a 2002 as they are bigger than the 03?????? They are convinced my 03 ( both I have 2 ) must have a different reed cage. I do have a 99 reed block also and it is smaller ( the reeds they sent me for my 03 fits the 99 cage )

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I forget which year the reed block changed.  I think it was 01 or 02 when they went to the narrow cage.  The parts I'm referring to is just upstream of that.  The reed cage stuffer and carb boot.  Its a pretty substantial difference from the 99 to the 03.  The 03 carb boot is something like 4mm wider in the throat where it mates up to the reed block.

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Gotcha, I have both pieces and will measure them. Not sure at this point if I will try any more or not. Got a call from my guy today (Saturday) my bike is ready. I was out on the trail on the 230 so I couldn't get there in time to pick it up. But he was impressed on how good it ran. He was pretty sure it was a bad idea!!!  He told me it was a little laggy coming on the pipe and that I should try going back to the 03 powervalve spring to let it come on a little sooner. It's killin me that the weather here is in the 60's and I can't pick it up till Friday

 I will have more details including ride report as soon as I get some seat time and full jetting specs for you guys soon.

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already had cometic build me a 0.10 thinner base gasket and a 0.10 thicker head gasket to lower ports and keep pump gas compression. I am a shorter light weight rider. When the bike comes on the pipe 90% of the time I lose traction. I was racing a kx/kdx hybrid before I went to the 250. I absolutes loved the kdx power curve but couldn't hang with the 300's in the open fields. The 250 is trickier for me in the woods as I have to stay just under the bike really coming to life. A 47 rear sprocket really helped this but I wont rest till I see what the 36 does. It may be a brilliant move or a waist of time. But I will be the guinea pig on this one!!!!! 

 

That's how I rode my 250's in the woods, in the meat of the power using taller gearing.  I really liked 13-48/49 on my 05. What carb are you fitting?  I'm pretty sure the 36 PWK needs spacers made to take place of the 38.  For more torque you can install a flywheel weight and reed spacer, maybe try one of those carb inserts.  Power wings or whatever.  The principle is spot on, never tried one though. I think I saw a company that made a 3 stage insert instead of the regular 2 stage.

Edited by Jeekinz

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 The carb came from a 99 125 I used for a hybrid. It looks identical to the 38 except for a smaller casted bore (just like the crf 230/xr 250 carbs). I have a TMX that came from a 02 125 that indeed needs a sleeve to make it fit. I will try it on my next hybrid a the carb side fits the kdx boot and the intake side fits air boot, but thats for later. Jetting ended up 145 main, 48 pilot, will probably need to go 45 as it warms up. Advanced timing to first mark, air screw almost 2 turns out. Hopefully I can get some time on it this weekend to see what it really is.Tried the flywheel weight and didn't care for how it took away from the really low rpms. Started at 13, turned it down to 9oz and honestly will prolly never install it again. Also I am running a auto clutch which really eliminates the need of a flywheel weight. Built a reed spacer and again it really felt like it hurt more than helped. Gearing is 13/48. Couldn't tell much difference between 47/48. I stayed with the 48 as that is what are spare wheel had on it.

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 I did forget to mention that I had to swap the TPS and Powerjet from my 03 38mm as the plugs didn't match my 03 wiring harness. I know alot of people just unplug them. But I really wanted to leave them. I plan on diving into the reed cage when I get home earlier. I do have both so that will be easy. Hopefully Boyesen sent the correct set this time. When I used to build outboards we would epoxy around the reed cages to add crank case pressure, but don't think I can in this situation. Outboard motors are so much easier to tinker with!!!

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 First short ride in the frigged temps. No real noticeable low end gain. Cleaner feel/sound off idle. Nice midrange, maybe a little less but had a real controllable feel. Had no issues getting it on the pipe, but it is cold and lots of good cold air. Hit was still there, but not as hard as before. Lots more manageable. Still didn't get a lot of ride time and probably won't till end of March. Three races in April so will have a good feel of it by then.

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 Finally, ride report. Slightly stronger bottom, lots better mid range. Nice hit that retains traction (I am 170#)  The 145 main is a touch lean for the current temps. Leaves the pipe almost too dry and slight grey on top of piston and plug. Stepped it up to a 148 till it gets summer time hot. To be fair it wasn't a massive change like I had hoped for. The most noticeable change was less hit up top. The mid is nice, but didn't add a massive amount down low as expected. All in all I am very happy with it and will not change back.

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Finally, ride report. Slightly stronger bottom, lots better mid range. Nice hit that retains traction (I am 170#) The 145 main is a touch lean for the current temps. Leaves the pipe almost too dry and slight grey on top of piston and plug. Stepped it up to a 148 till it gets summer time hot. To be fair it wasn't a massive change like I had hoped for. The most noticeable change was less hit up top. The mid is nice, but didn't add a massive amount down low as expected. All in all I am very happy with it and will not change back.

I had a 03 with a PC platinum pipe. Very little low end. I have a 99 now that came with a 04 OEM pipe. I recently smooshed it on a Bad log crossing. Purchased a lightly used FMf gnarly and tc2. Bolted on without jetting changes (stock) and the result is great!!!! I've used gnarly on yz (no low gain and too snappy) and a cr (no real change) and now this 99 kx. Didn't really expect much but was very happily impressed. I know your 03 cylinder is different than my 99 but you never know. More torque and grunt and harder hitting mid range. Didn't notice less top end power. Definitely a more lively (while usable) motor now.

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