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Re: Rookie question: Dashpot Oil

To: "John Roberts" <robe3099@bellsouth.net>, "Spridgets"
Subject: Re: Rookie question: Dashpot Oil
From: "David Lieb" <dbl@chicagolandmgclub.com>
Date: Sun, 22 Apr 2007 08:33:14 -0500
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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