H4 Howard wrote:
<snip...my SU carbs are vacuum controlled..that means
that they react to engine vacuum to set the needle height in the fuel
jet..Less vacuum means more fuel, a good thing..acceleration mode is
called...snip>
Howard,
The vacuum signal that controls the SU slide and needle position is taken
from the base of the dashpot slide right over the jet. This keeps the
pressure drop across the jet (from float bowl to carb throat) constant.
This is outside of the throttle valve so it is isolated from actual
manifold vacuum.
The dashpot slide works on the variable venturi principle. The slide has
three distinct pressure areas. The smaller cylindrical part that you see
inside the carb reduces airflow area under it and over the jet. Whenever
air flows into the engine, the air has to speed up under the slide and, as
Venturi predicted, the pressure drops. So the bottom of the visible slide
is at low pressure compared to atmosphere. Inside the dashpot, the slider
has a larger disk that fits closely to the dashpot. The lower surface of
this disk is vented to atmosphere so you have a ring of surface that is at
a higher relative pressure than the jet area. Now here is the trick. The
slider is hollow and allows the vacuum around the jet to suck air out of
the upper chamber. So you have the whole surface area on top of the slider
at the lower jet pressure while the bottom of the slider has the jet
pressure acting on the middle and higher atmospheric pressure acting on the
ring shaped area of the sealing disk. The slider moves until all forces
are equal and sets the fuel mixture.
Acceleration enrichment is aided by artificially holding the slide down and
increasing the pressure signal across the jet during increases in air flow.
The dashpot damper does this job.
As far as a ZS carb goes, the diaphragm is used on the dashpot as a
replacement for the close tolerance fit between slide sealing disk and
dashpot on the SU. On HD carbs the diaphragm takes the place of the choke
mechanism on their H counterpart and raise/lower the jet as a function of
manifold vacuum. So the ZS uses a diaphragm to eliminate a close tolerance
fit and reduce costs. The HD adds a diaphragm to give more function to the
carb. On the ZS used in late MG's, there is a "power valve" on the side of
the carb that performs a similar function as the HD diaphragm although I
don't think that would be as effective.
>From a theory standpoint, the three most important variables for choosing a
fuel mixture are air consumption, engine load, and temperature. All SU's
(and ZS AFAIK) handle the air consumption portion fairly well. The HD and
late ZS carbs also use manifold vacuum as a measurement of engine load.
Earlier Middle ground ZS and later SU's use bimetallic springs in some
fashion to compensate for temperature. If SU had just included all three
mechanisms in one carb, then the late seventies may have been less
traumatic for our LBC's. FWIW, Honda used dual ZS knockoffs until at least
1988 on their entry level Preludes. I never took mine apart but I would
bet that these carbs included all three compensation devices.
Regards,
Bill Eastman
61 MGA with nice simple carbs
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