The SU is also referred to as a "constant velocity" carb. Not exactly
correct but the velocity and "signal" (vacuum at the jet) are fairly constant.
The
piston acts like the mass air flow sensor and the needle is like the fuel map
chip in a modern fuel injection system. Because if it's ability to measure
air flow the SU is less sensitive to atmospheric changes such as altitude. This
renders the question of "what is a good high altitude needle?" fairly moot.
When I started flow bench testing SUs I saw something interesting. With the
throttle wide open, slowly bring up the manifold vacuum. Nothing much happens
till a certain vacuum level is reached. At that point the slide goes from
fully closed to fully open almost at once. It is trying to maintain that
constant vacuum level and pretty much switching from closed to open when that
threshold is reached.
keith williams
In a message dated 6/6/2006 3:56:01 P.M. Pacific Standard Time,
Steve@olsonarchitect.com writes:
Does this mean that the air velocity is maintained constant by the rising
piston in the SU carb?
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