Idle screw question. Rich or lean?

Also, let me get my timing light on it to confirm tomorrow also. I know its a bit more than the factory 10*, but not a lot. I wanted the small idle speed increase on the old and tired reverse idle screw 4160 without opening the butterfly more.


Why bigger power valve?


The number doesn’t represent a bigger power valve, it sets when the power valve opens relative to manifold vacuum.

The higher the number, the sooner it opens. Late opening power valve timing (relative to manifold vacuum) is a tuning disaster.