It's been around for a while. To explain what it does, I'll explain what it is. It is a hollow tube which you mount in your carb bore. On the outside there is a small box into which the tube mounts. The box is a little carburetor. It has a tube where you supply fuel, and it has a series of holes where you put an air filter. The 5-position dial adjusts which air hole is active, and this controls what ratio of air-fuel it delivers.
What it is supposed to do is provide an atomized air-fuel mixture which supplements your normal jetting. I think the numbers they give out are "up to 15%" replacement for your normal mixture, so you need to jet down a little bit to compensate. The concept behind it is that it provides a level of atomized fuel which is more immediately available, lighter and quicker to react than the normal carburetor circuits, so it tends to help acceleration and fill in the transition areas of the carburetor.
My desire with the CV carburetor application is to use it as a pseudo acceleration pump. If placed correctly, where it will react directly to an increase in signal, it should supply an air-fuel mixture as a stop-gap while the slide is equalizing. I don't believe I have placed it in the optimal location yet. I didn't feel like drilling holes in my carb.
In my pictured install, I'm assuming that it is just supplying a general whift of highly atomized fuel in the vacinity of the carb intake. It would be interesting to experiment with different nozzle locations to see the affect. I installed one on my two stroke before but I wasn't so impressed with the results. Yes it supplemented (and in fact I could get it to detract in the lower settings) the air-fuel ratio, but my two stroke didn't give a squat about atomization levels.
The dial-a-jet differs from a powerjet in that a powerjet will supply only fuel. And a powerjet requires a significant vaccuum signal in order to siphon. The dial-a-jet has a small chamber in the box where air & fuel are mixed, so what gets drawn through the tube is a lighter mixture and requires less signal to siphon. When I hooked up a powerjet to my two stroke, it only became active at near full throttle. The dial-a-jet hooked on the same bike, same hole in fact, was active everywhere and in fact I had to lower the needle.
The dial-a-jet is a really simple device. I find the construction a little cheezy but I got some cheap ones off rockymountatv that were returns. If you were to make one yourself, you should make the chamber accept some standardized main jet rather than trying to make a funky dial. Also there may have been something mentioned in the instructions about maximum fuel line length. I imagine that having the chamber too high above the float level is probably not a good idea.