The document that Dar supplied the link to is curious. If you bothered to read the WHOLE THING, you would see that only the TN instructions are worded that way. Why? Well the individual putting together the pretty PDF document might not know anything about carbs and didn't question the text given. It could be old text that nobody bothered to review and there was a issue in translation years ago. Rather than wasting your time here, one might write to Webra and question their manual.

Both of you are correct. The low range certainly is influenced by the high speed needle. Make a drastic change to the main needle and it will change the "mixture control"(idle) needle. But it should be noted that Dar is very correct that you would not use the main needle to adjust the midrange (part throttle) running characteristics. Very few of our carbs have a midrange adjustment so we are stuck with what the manufacturer supplies as a tapered needle to control midrange and hope they did things correctly. If the the taper and inward movement of the barrel is correct will properly meter midrange, in conjunction with providing a good idle mixture. The taper on the idle needle does two things.

The taper on the low speed needle is chosen so that it:
A. Allows adjustment of the low speed mixture strictly by means of turning the needle in and out like you do when you adjust it.
B. Meters fuel in the mid range by the action of the barrel or slide moving the entire low speed needle toward the spray bar orifice.

Now, it should be apparent that adjusting the low speed needle influences the midrange mixture. It should also be apparent that the taper on the low speed needle influences the midrange mixture and is dependent on throttle setting. No manufacturer offers different tapers for the low speed needle, because good enough is just that. If you don't understand what I mean maybe you could look at [link=http://www.mikuni.com/fs-manuals.html]Mikuni HSR Tuning Manual[/link]. Here it is obvious that there is a separate high speed, midrange, and idle circuit. You can adjust the midrange independently by changing the metering needle or adjusting its position. Our carbs don't have this.

I recently installed a Rossi slide carb on a diesel conversion I'm working on. It's a two needle setup and works just like I described above. The high speed needle is easy to set. The idle to off-idle is set nice and lean. The midrange is obviously rich and smoky. Why? Simply because the taper on the low speed needle is incorrect for use with fuel at nearly 15:1 air fuel ratio compared to the 4-5:1 used in most glow car engines at 30% nitro. There is no adjustment on the carb that will correct for this. I need to make a new needle with a appropriate taper to properly meter midrange.

Now, the TN carb I have seems to run on the main needle until about half of the area of the throat is closed. That seems to be more than others I have but similar to a couple car slide carbs I have. Keep in mind though that a reduction of half of the carb area is still a lot of power, probably 75%, not half RPM.

I'll have to agree that the wording for the TN models appear poor, but do we really know that this isn't how it is designed to work? It's been a while since I ran my Webra T4-40, but I had no trouble tuning it when I did. I used regular procedure tuning the main needle first, then adjusting the low speed until I had a good transition. Looking at the carb I knew this method would work and it did.

I was going to bring up Saito carbs last week. If you don't know that the low speed needle influences the main, you will have fits tuning a Saito. I noticed it when I was tuning the the FA-80 I own. Funny how the manual makes no mention of it.