John, and group---There are three "adjustments" provided on the ZS carb
that sometimes get crossed up. (Me included) They would be the temp.
compensator, the bi-pass valve and the idle trim screw.
1) The "compensator" lets air in without crossing over the bridge, when
the bi-metallic blade senses hot engine conditions. This is supposed
return the mix to normal.
2) The bi-pass valve lets air in when the throttle plate is closed and
manifold vacuum is high enough to pull the valve off its seat. It
appears to me that this would also draw in fuel, as this air gets pulled
over the bridge. The book on the ZS writes that it "admits air". Other
than lowering intake manifold vacuum by a few inches, I'm now not sure
how air alone would lessen any "popping" due to lean mix. Maybe Richard
knows the physics involved here?
The Idle trim screw also lets air in thru a port starting at the inlet
flange. (sort of like the air supply for the temp. comp.) Its purpose
seems to be just for sensitive tuning, when it is known that the
air/fuel mix can use a finer touch than afforded by adjustments to the
needle heights.
All of the above can be reviewed by referring to pp. 38-39 in the Haynes
ZS Owners Workshop Manual.
Dick
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