Needles
 

I had my carb opened the other day and was comparing the stock needle to the one in the ltr kit. Lots more taper on the stock one. I'm curious what the effect of the taper has on carburation/hit/juice to the rear wheel. I realize more taper means more fuel which explains the rich stock setting. Why not just drop the needle down a slot?? I was tempted to put the stock needle back in just to play with it. Word up on all this taper stuff????

There is a lot more to it than that, but basically it boils down to the clearance between the needle OD and the nozzle ID at a given slide position that determines available fuel. The straight diameter affects this off idle before the taper comes into play by reaching the nozzle. The slide cutaway affects airflow at a given throttle opening (small openings). Ideally all this is balanced and if it can be power will be smooth and clean. If not, you can be rich in one area and lean in another. The stock needles are actually triple tapers (angle changes three times over length). The more narrow tip means that the overall fuel flow can be more, and will generally require a leaner main jet than a single taper needle.

From my experience, if the slide is wrong its difficult to dial in settings with any needle. Jetting three different 250s, the #7 slide is much eaiser to work with than the #6 at 0 - 1000'. For a 300 I'd try a #7 vs #8.

Regarding your last comment about the 300. Why would you recommend the #7 vs. #8...just curious. I've seen others mention that the #8 helped clean-up the low throttle bog. Cheers

Sorry, me bad. It was a typo and I ment a 300 should end up with either a #7 or #8, vs. #6 or #7 on the 250. Probably the #8 as the 250s like the #7.

I have never personally jetted a 300, so this is pure speculation, but I would get an #8 to try if a few needle swaps did not provide good results with the #7.

my 06 300 came with a #7
Never had a low throttle bog.

stock needle sucks...
 

I have an 06 ec300 with the stock needle top clip position, 180 main, and 38 pilot. stock pilot (40) was too rich (airscrew at 2 3/4 turns out and still rich).
changed the needle to second from top clip position and topend is now close, but mid (1/4 to 2/3 throttle) seems to be rich (i.e. burble).
I have ordered the LTR jet kit and I hope it cleans up the confusion that is "my jetting".
I had some crazy problems with my 03 ktm 200exc, but the jd jet kit needle worked wonders to smooth out the carb at all throttle openings.