> I just bought a 78 MGB and it has a single Zenith carb. I know nothing
>about this carb! I asked around where I work (we have 6 full time
>mechanics)and noone had any idea what I was talking about! Whats the deal
>with these cards? The resevor at the top with oil(?) and a pistion? Any
>explenation of the basic workings would satisfy manys curiosity! thanX
They're what's known as 'constant depression' carbs (hence the CD in 175CD).
As Bernoulli will tell you, air pressure decreases with increased air
velocity through the throat. This pressure is ported to the top of the
piston, while the piston bottom sees only atmospheric pressure. Therefore,
increased throat air velocity causes the piston to rise, expanding the
throat area, reducing air velocity and increasing pressure again. When the
net upward force on the piston from the pressure difference between the
throat and atmosphere matches the downward force on the piston from
compression of the spring (did I mention the spring?), the piston stops moving.
At the bottom of the piston is a tapered needle which regulates the
volumetric flow of fuel through a jet. The higher the piston, the more jet
area is exposed, and more fuel gets introduced into the airstream.
The oil in the damper of the piston prevents the piston from rising too
rapidly when the throttle is opened, allowing the reduced pressure in the
throat to suck more fuel from the jet, which enriches the mixture for
acceleration. The oil-filled damper basically performs the same function as
the accelerator pump on a conventional carb.
(At least that's how I figured it all works.)
It all makes for a most ingenious and fascinatingly different fuel-metering
method.
--
Jay Tilton | jtilton@vt.edu
|