I tried something I haven't read about. I'm way too cheap to buy the Dynojet kit so I tried something a little different. I bought the "Allsballs" KLX300 carburetor rebuild kit #26-1133. It comes with everything to rebuild a klx300 carb including needle and clip but NOT the collar #92143-1667. It does however come with a small thick rubber oring #92055-1245 (92055a). I got tired of waiting on the collar to come in the mail. This little oring slips perfectly over the top of the needle and clip. I put the clip on the top groove, installed a 150 Keihin jet and kept the 40 pilot jet , 2 turns out on air (fuel) screw FMF Q4 with baffle removed. I took it for a test ride and let me tell you all, it feels like a totally different bike. A HUGE increase in overall power and I feel like I have a bigger rpm range for maximum torque.

 

I hope by "top groove" you mean that you've made the needle "stick out" of the bottom of the slide as much as possible - made it as long as possible - put the clip most toward the blunt end, not the pointy end... ? If not then you need to do so before doing anything else. Hopefully you (or a previous owner) haven't monkey'd with the stock slide spring or the slide lift port - both must be stock.

Given the above is true, you are ready for your next performance leap, it's very simple, install a K158 main jet and pull off the airbox lid.

Keep at tight grip for the first few snap-n-hold-WOT-to-redline ...

 

My air box lid is already off and yes by top notch I mean the thicker end. Will a 158k be too big without a aftermarket header? I'm about 400-600'asl. I gotta say was a real skeptic going from a 125k to a 150k main jet but now I'm a believer.. Thanks klxster. I have learned a lot combing through your posts..I don't think I'm even going to mess with the needle collar when it comes. I'll keep it as a spare.
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I got to ride the new setup a little more today...I am VERY impressed with the performance...very smooth, no popping on deceleration.

 

My big question is what's difference does it make between different thicknesses of shims/collar that sits on top of the needle. I just took a chance with the oring that came in the rebuild kit which goes to a totally different part of the carb..... My hat off to klxster.. I would have never imagined going that much up on jetting etc without reading all his posts

 

Thanks 1wheel, but look, with the K150 main jet in use, you cannot remove the airbox lid as the engine would run very lean.

 

Ok klxster. I was just following what you said in your thread about mcm vs no mcm dyno and you said to run lidless with dj140=152K .The biggest jet I have is a 150 so I figured it was close enough. So you recommend a158k with 40 pilot and no airbox lid?

 

You don't own a KLX250 - so you cannot use my CVK setups verbatim.
The 300 cc engine requires at least an increase of 2 jet sizes from the 250. So, if you are running the stock header, the lidless CVK recipe includes a DJ144 / K158 main jet.

Using the accurate jet size chart ( link is on my signature line ) , your K150 = DJ138. This small main jet is only useable if the airbox lid is in place choking off airflow ..

Also, the recipes for the KLX250 and KLX300 are identical except for the main jet sizes required. For instance, never change your clip setting off the top-most notch.
It appears your kit gave you a N1TC needle - which is what you are using now. That needle is too short and barely controls mid-range fueling in a lidless setup, but with a stock slide spring and stock slide lift port, it does an acceptable job.

 

Klxster I think there's is a misunderstanding. I have a 2007 klx250 not a 300.. what exactly is the best carb setup with a slip on exhaust for a klx250?

 

Ok then..

K150 will run with a lidless airbox without causing engine damage. The AFR will be a little too lean for max power as you are running a little over 13:1 AFR @ WOT.

The K152 is the absolute leanest jet for a lidless airbox - and for those running the DJ needles, going to a K155 would likely make a little extra power over what I made..

With your N1TC needle, a K152 is about as large a main jet as you should run. The N1TC cannot control midrange fueling as well as the DJ needles do, causing a richer midrange than the DJ's..