"Mark DeHaas" Scribed:
Subject: Fuel mixtures
>
> From my experience engine idle fuel mixture and steady state fuel
> mixture ratios are very different. When an engine idles it idles on
> the rich side. When one is cruising on the highway the fuel to air
> ratio is very lean. So one can't assume that if the idle mixture is OK
> that acceleration mixtures and steady state mixtures will be OK or
> visa versa!! From what I understand SU and Strombergs rely on engine
> vacuum to adjust fuel to air ratios under acceleration.
Go read the book again. (:-)) These carbureters are known as "constant
depression", which means that the vacum in the venturi is kept pretty
stable, by varying the size of the opening via the carb piston. The mixture
is changed by varying the size of the opening in the fuel supply jet
by means of raising and lowering a tapered needle in a fixed size hole
(the "jet"). The profile of the needle is selected to provide the desired
mix at any given throttle opening. Normally, the stock needle gives a
very similar mix throughout the range. The "standard" lean needle is a
tad fatter in the operating range, allowing a leaner mix for better economy,
just a rich needle is a bit skinner in the right place, allowing increased
power and fuel consumption.
The choke method on an SU is to simply lower the jet from the needle
while leaving the air opening alone. This results in a larger fuel
supply opening, and a richer mix.
It is even possible to create compound tapers, say to give a lean, fuel
effecient mix at part throttle (cruise mode), and tend towards the
rich side at full throttle (accelleration mode).
________
/___ _ \ Roger Garnett (sggy@crux1.cit.cornell.edu)
/| || \ \ Agricultural Economics | South Lansing Centre For
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