MSN = Mixture

It controls the air/fuel ratio.

The tapered end of the LSN (a needle valve) fits into the open end of the MSN, sometimes called the spray bar, while fuel passes through the MSN and around the LSN.

Adjusting the MSN moves its end position/spray bar relative to the convex shape on the slide valve (this is where air mixture changes)... as well as in relation to the LSN (this is why fuel mixture changes). Hence, the MSN is always a secondary adjustment! You must have the HSN and LSN set prior to the MSN. Furthermore, even though the mechanics of the MSN take place at low speed settings, the results produced are seen in the midrange... which is why the mixture is sometimes mistakenly called the mid needle.

The original VZ was designed to run 8 and 9mm inserts. Running smaller than that closes the tuning window (I've seen people running a 6mm venturi).

Of course the best way to get run time is run a larger venturi and learn throttle control.

Unfortunately, there is no set formula or magic guide on how to tune the MSN; but you can use the MSN in your favor once you understand how it works... You can richen the mixture needle a touch when running a small venturi, lean mixture on colder days when air is dense, etc.