Being a 250S noob, I'm assuming that N1TC needle is the OEM?
Anyway, when ever we would drill out a slide, it was to improve throttle response and smooth out a snatchy throttle. We never used aftermarket and we always used a washer. I'm not sure if this bike has snatchy throttle response in stock form, or even with the free flowing air system and exhaust. I'm sure using the stock needle will help you calm your jetting down. I don't know why you'd use an aftermarket design if it doesn't have a different taper and doesn't sit in the carb the way OE needles do. CV carbs are pretty straight forward and not as touchy as other designs, so you should be getting close. Keep it simple.
I'm glad that I've been able to follow this thread because it seems like the group has done a great job in helping and you've done a great job working on the problem.
Sad that so much has to be done to undo what emissions regulations have done to bikes.

 

I believe the N1TC needle is the OEM needle for the KLX300 model...not street legal models.

 

Why not use the stock needle? Isn't it adjustable?

 

No, OEM is not adjustable. I put a #4 flat washer under the stock needle to shim it and took the N1TC out. Many clearly highly recommend the N1TC, I just don't see how it works with being so much shorter. I don't know how some people are not running extremely rich with it.

 

Not sure why the KLX300 needle would be needed unless you bored, ported, camed and removed the air box. The small adjustment most bikes require to open up flow should only require a shim under the stock needle to help under revs. That, along with moderate jetting changes to main and pilot should be enough. Most bikes are usually geared a little poochy to get them through testing, too. I'd go down one tooth on the CS once you get your jetting right, but that seems like an awfully small sprocket to start with. Looking forward to hearing how your efforts turn out.

 

I have a 13T CS ordered.

 

Looks like they're not hard to swap back and forth without too much issue.

 

There are a couple tricks to simplify.
I change them regularly. 13, 14 & 15.

 

The N1TC needle is the 49 state needle for the KLX300. The non-adjustable needle started in California, for emission requirements, even off-road. If the KLX300 is opened up for better air movement in and out of the engine, the non-adjustable needle would cause a lean engine miss fire in the mid and upper RPMs.

Ride on
Brewster

 

Cool. Why would he need to open it up that much for an air filter and silencer? I really am asking because of experience on other bikes and sincerely wonder why so much adjustment to the bike is needed for a simple mod? I have to take the advise of the guys with experience on this model from here. When I do mine in the near future, I'll use great care from the results here.