Ok, I've completely lost track of what is being argued here (forget the fact that my thread was about pumps with non-pump carbs).

The majority of two-needle r/c carbs that I own work in a linear fashion. That means that the fuel exits the tank along a single line and enters the carburetor through a nipple located along the tapered portion of the high speed needle. The fuel then travels past the HSNV, through the spraybar and exits into the barrel either through a slit or hole of some sort. This opening in the spraybar where the fuel exits is regulated by a second needle. Sometimes this needle is called an Idle Needle, Mixture Control Valve or Low Speed Needle (among other things).

The point is that fuel travels in a linear path. It passes the HSNV first and then travels past the Mixture or Idle needle.

Every single molecule of fuel that exits the spraybar is first regulated by the HSNV because that's where the fuel tubing attaches to the carburetor!!!

Now, this regulated amount of fuel - which has been regulated by the HSNV - then gets regulated a second time by the Idle or Mixture needle. However, this needle's setting is dynamic, not fixed like the HSNV. As the throttle is rotated the amount of regulating this needle performs is increased or decreased. But, it is always regulating fuel that has already been regulated because the fuel travels in a linear path.

To put all this in the simplest terms I can:

The HSNV controls the amount of fuel that enters the spraybar.

The Idle or Mixture needle controls the amount of fuel that exits the spraybar. However, it alters the amount of fuel that exits the spraybar in relation to the amount of air the barrel allows into the carburetor. It's purpose it to maintain a constant ration of fuel to air. It can never let more fuel exit the spraybar than the HSNV has previously allowed to enter the spraybar!

To sum up:

The HSNV is the "master valve" that determines how much fuel can enter the carburetor from the tank in the first place.

The Idle or Mixture needle controls how much of this pre-regulated fuel the engine will receive. And, this amount is increased or decreased depending on the amount of air that is being allowed into the mix.

Here's a good analogy: Think of a transmitter and servo operating a control surface. If you program your transmitter's EPA to allow 100% travel then the servo can rotate fully. If you set it to 75% then the servo can only rotate 75% of the way. This is the HSNV controlling the overall amount of fuel entering the carburetor.

Now, imagine moving the stick on your tx. This is the throttle/barrel mechanism of the carburetor. The stick controls the actual amount that the servo will travel - up to the maximum amount that your EPA allows, but not more. Even if you move the stick all the way, it can't move the servo more than your EPA (HSNV) will allow.

The stick moves the servo within the range of travel permitted by your EPA setting. Likewise, the Idle/Mixture needle in conjunction with the throttle/barrel determines the amount of fuel exiting the spraybar, but it can't exceed the amount of fuel the HSNV is permitting to enter the spraybar in the first place.

David