On Fri, 2 Apr 2004 13:59:49 +0000 (UTC),
wrote:

OK... I'll concede that I oversimplified a bit.... The air/fuel ratio (as
in mass-air/fuel, like 12.5:1, 14:1, etc) does change when the throttle is moved.
Even without changing altitude, if the mixture is set to peak EGT, for example,
increasing MP by an inch will lean the engine (unless it's starting to enter the
full-throttle fuel enrichment regime). It's probably somewhere between the two
extremes (absolute fuel vs. relative fuel).


Actually, you hit it pretty close. You do need to remember that aside
from the mixture control (and probably a crude economizer circuit), we
are talking about a one-barrel gasoline tractor carburetor.

On a MS carb, the "mixture control" is a chunk of brass on the end of
a two inch piece of flex cable (similiar to a tach cable). The mixture
knob rotates an arm clamped onto the top of the cable, rotating the
piece of brass on the bottom.

This piece of brass is stuck inside a brass cup on the bottom of the
fuel bowl with a "window" in it. At ICO, the window is "shut", at full
rich, the window is wide open.

The fuel gets sucked by the venturi out of the bottom of the bowl,
through the window, and out the main metering nozzle on the carb into
the induction airstream. The combination of the venturi type and the
main metering nozzle determines how much suction (relative to the
ambient pressure in the fuel bowl and the float/fuel level) is
generated "through" the valve.

Throttle plate position/engine rpm will indeed vary airflow through
the venturi, and consequently will affect the amount of suction. But
as you've indicated, the mixture control is a manually variable (with
the mixture knob) mechanical restriction to this suction. There is no
mechanical interconnect between the mixture valve and the throttle.

TC