The engine will run best in temperatures above freezing once it reaches operating temperature with the flapper closed.
Before the engine reaches operating temperature in any climate (unless you live where the ambient air temperature is 180 degrees) it will run best with the flapper closed.
In frigid sub-zero climate, the engine will likely run best in the middle position.
It would be handy if you could change the setting on your flapper without any tools when you start the engine first thing in the day.
Then you could easily set the flapper to the open position which diverts heat to the intake, thus preventing raw, unburned fuel from leaking past your rings, scrubbing the oil from the cylinders, causing premature wear, and then contaminating the crankcase oil causing premature wear to the bearings.
Once the engine reached operating temperature, you could just move the lever to closed and roll on.
Short answer to your question is leave it in the closed position except for cold starts.
I find it puzzling that they would make that setup to require tools to change the position of the flapper.
What is the lever made of? If it was spring steel, you could back off the set screw to where the spring steel lever would allow the the lever to move by hand, but still fall into the detents for the 3 positions and stay put.
