MJN Carburetor - Theory meets Practicality. Fujio Yoshimura figured out a way to create a Multiple Jet Nozzle that would be more efficient at atomizing fuel. It took a lot of engineering to develop and test the initial prototypes, but once dialed, the results were spectacular.

The key is a nozzle which replaces the standard tapered needle. The multiple jets spray a fine mist of fuel, which atomizes much better, thus creating a more efficient burn. In addition, the atomized fuel is centered in the bore of the carburetor thus taking advantage of much higher intake velocities creating even more power and efficiency.

Benefits of the MJN: MJN system atomizes fuel much better than ordinary solid type needles. The MJN nozzle controls the amount of fuel in proportion to the throttle opening (intake air) by accurately drilled and positioned fuel intake jet orifices. The resultant more efficiently atomized fuel gives better throttle response as well as better fuel efficiencies overall. Also, the MJN system supplies the fuel through the MJN nozzle in the center of the bore where air speed is the fastest. This atomizes fuel even better!

Another very important theory of MJN system is when the throttle is shut off and engine braking occurs, very minimal amount of fuel is guided into the intake ports. Therefore, remarkably crisp throttle response is obtained, upon corner exit and transition back to 100% throttle settings
(I copied the above statement from the YouTube clip)
https://www.youtube.com/watch?v=1HgXsgHSBbY

So its not worth the $400 plus dollars? I should stick with the Flat slide?

Hmm that's interesting Dave. So the three main differences are size and cost. Also the flat is a little more responsive due to having less travel under the flat slid compared to the round slide.