Hi Jerry,

No expert on these things, but I would think it is because the load on the engine is constant on the bench, while it changes a bunch in the air. 

When you run an engine on the ground it is loaded more.  The prop pitch is "true" when static (no forward movement).  As the prop moves forward it is actually reducing pitch relative to air speed, which reduces the load.

So, when it is running on the bench it is actually cavitating and creating turbulence which is drag.  When it is in the air it is penetrating like a screw.

That is why a properly setup stunt engine (of any kind---including a 4 cycle) should richen slightly after takeoff.  If it goes the other direction, it is starving and going lean.  You typically see this on profiles that are sucking fuel uphill against centrifugal force (leaning after takeoff).  The Paul Walker style Saito run also did this.

To get a motor on the edge of the 4-2 break it may have to run much harder (mostly 2 cycling) on the ground.

Wish I could tell you more, but my Grand Dad had the Engineering Degree from NC State, not me!

The load on the engine is different while it is stationary vs. in flight.  The changing loads in flight are what causes the 4-2 break............... I guess!!   

I've noticed with all my .40 and .46 size engines, that they have a 4-2-4 break on the test bench when breaking in, at almost any needle setting.
However I spent a lot of time to get the right needle setting on the ground to have it 4-2-4 during flight. Any ideas about why that is?

Thinking back over this, I think you really want to know if everyone takes a lot of time finding the "needle".  All stunt engines will run right on the bench and different in the plane, according to the needle settings.  Some more than others.

In the beginning, i had to fiddle quite a bit with each one getting to run in the air correctly.  After a while, I learned what the correct sound was and got the needle to that.  then it was usually a couple of "needle" flights and tank trim (maybe) to get the run near to where I wanted it.  I made two "needle" flights on my new Vulcan.  I took out about 1/2 the fuel for those.  First one too rich, next one closer, then the third I put about one more "click" on it and filled 'er up.  Now I will just need to maybe get a slight turn next time out, or when I go to another location.  "getting a needle" is sort of a learned experience.  The bench is really more for just breaking an engine in and learning a little about what it needs to get started quickly, IMHO.  Not where you get your flight settings. 

Another thing to remember, if the engine is propped close to correctly, it wil richen after it gets in the air.  The prop "unloads", thus the engine goes richer.  I have always referred to the proper ground setting as a "cackle".  The engine is just rich enough to not stay in a solid 2 cycle, but it's not in a rich 4 cycle.  RFandy Smith is always saying that his engines should be in a hard 4 cycle in less than a lap from launch.