TVILLIANCOURT/anybody

How does your "pinch test" work?

"With the engine idling pinch the fuel line and count the seconds it takes before the engine starts to change pitch and die.
Less than 3 seconds the engine is running lean
more than 5 seconds the engine is running rich."

Is the length of time the motor runs a function of how much fuel is already in the carburettor/engine? Is it a rule of thumb that applies to most 30 size engines?