During an engine restart procedure, what are the purposes of a.) throttle back to idle and b.) primer "in and locked"?
Source

Idling, or closing the throttle, might help break up any ice that may have formed and caused the engine to quit. That's why it's followed by Carb Heat On.

Also, a failed engine (of any type) is best restarted at idle power. Otherwise the extra fuel will either flood or overheat the restart. But nevertheless, always stick to whatever the checklist says.


  Throttle ice forms when your throttle is partially closed, typically between cruise power and idle. As air moves through the the Venturi in your carburetor, it decreases in temperature, condensing water vapor from the air. The water then starts freezing to the carburetor parts, restricting airflow. Eventually, you'll start losing RPM or manifold pressure, your engine may start running rough, and if the ice gets bad enough, your engine will quit.




If a primer is inadvertently left in the pulled / unlocked position, it may flood the engine with fuel.