Begging to differ, the greatest change is in the annular area (between
the needle and the jet) available for fuel to pass through into the
airstream (which is significantly below atmospheric pressure.) The
pressure change of 1/8 inch of fuel is approximately equivalent to
0.075 inch of water, about 0.14 mm of mercury. Totally
inconsequential. During operation, the depression across the venturi
in an SU carburettor is about 2 inches of mercury, so the resultant
error is 0.3% due to a fuel level error of 1/8 inch. And lower is
safer, though there is less reserve in the bowl, then.
Float level is problematic if the fuel level is set too high, though.
Then at shutdown, when the fuel in the lines heats or the fuel pump
spring is holding pressure on the fuel, the fuel will flow into the
venturi and flow downhill from there, either into the engine (causing
flooding) or into the air cleaner (causing fires.)
Donald.
> Date: Fri, 01 Sep 2000 14:25:25 -0400
> From: "Nolan Penney" <npenney@mde.state.md.us>
>
>
> When you adjust the mixture on an SU, you do it simply by raising or lowering
>the jet.
> This does two things, it changes the needle relationship, and it changes the
>liquid
> fuel position in the jet. The engine is drawing a very small amount of
>liquid through
> that jet, so tiny changes in that circuit have great effects on the overall
>fuel
> mixture and how the engine runs.
>
> Moral of the story? Fuel float settings are critical.
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