View single post by Mark Rosenbaum
 Posted: 04-22-2007 02:16 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
smcmanus wrote: [...] I thought that a lift of the vacuum valve would cause the mixture to richen. 

That issue has tripped a lot of folks over the year, myself included.  Fortunately, a sharp-eyed reader caught my error and called me on it, some time back.

In a Stromberg, the vacuum valve and the jet bridge form a variable venturi whose size changes depending on how much the vacuum valve is lifted.  Fuel delivery to the airstream flowing into the engine depends first on the speed of the air flow over the  bridge, and second on the amount of fuel exposed to that air flow (i.e. the area of the annulus between main jet and mixture needle).  When you lift the vacuum valve without altering the throttle setting, the air speed across the bridge decreases more rapidly than the annulus area increases, and the mixture becomes more lean.