Back Suction Economizer / Power Enrichment

 

Description of Operation

This is the best explanation for why my mixture leans out when closing the throttle.  This is normal and supposed to happen.  Back suction via a port in the throttle body near the venturi reduces the pressure of the fuel near the base of the main jet when the throttle is in middle range.  At wide open or idle it is not in effect and the bowl pressure is near atmosphere.  There are also some air bleeds near the jet that lean the mixture at partially closed throttle.

So the fact that the mixture goes a bit lean when closing the throttle from cruise is entirely normal.