John,
You have probably noticed the apparent absence of an accelerator pump in the
rather simplistic functionality of the SU carburetor. This is not because
these engines have so much torque that they do not need one, rather that,
like so many other things on a "normal" carb, it is disguised.
The function of an accelerator pump is to enrich the mixture when you hit
the happy pedal and want the car to go faster. Holley carbs, Rochester
carbs, Weber carbs, etc., do this by shooting a jet of raw gasoline down the
throat of the engine. A bit crude, poorly atomized, and marvelously
effective.
SU carbs, OTOH, use a different approach. The dashpot, under normal
operation, regulates the size of the venturi for the amount of air that the
engine needs. When you accelerate, it needs to open further. Until the
dashpot gets to the new height, the actual amount of engine vacuum at the
jet is higher than it would be at steady-state, so it sucks more fuel in,
yielding a richer mixture. How long it takes the dashpot to get to the new
height is a factor of the viscosity of the oil you poured into the top of
the dashpot. Hence, a heavier oil gives you a richer mixture longer during
acceleration, while a thinner oil gives you more air sooner, but less of an
"accelerator pump shot". As Bill indicated, "if your car stumbles when you
hit the gas, . .. go thicker.. .. if the accelteration seems sluggish, try
thinner."
The dashpot oil also serves to dampen the effect of the vacuum pulses from
the engine so that the dashpot height is not jumping around. This is
important to those of us not racing around at wide open throttle the whole
time. Worth checking the level periodically.
David Lieb
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