>The thinner the oil the quicker the piston moves up which takes the
needle
>with it, thus moving the lower thinner part of the needle to the fuel
>dispersal area of the jet providing a larger orifice area
Half correct -
The oil weight has (essentially) no effect on how high a given amount of
vacuum will lift the suction piston, only how fast it will lift it.
Remember the way an SU or Stromberg (CD, Constant Depression - in
American English "constant vacuum") carb works is by varying the venturi
size so as to keep the velocity of air moving through it constant. Thus
the vacuum seen at the fuel nozzle is also constant. The exception to
this is when the engine needs to accelerate. In order to accelerate an
engine, the mixture needs to be richened (that's why other carbs have
accelerator pumps). In the SU type carb what happens is the damper
oil's resistance causes the suction piston to rise more slowly than it
otherwise would which (bertouli's principle) causes the speed of the air
moving through the venturi to increase momentarily. That increase in
velocity causes a correspondingly greater vacuum at the fuel nozzle
which richens the mixture until the point at which the suction piston
reaches equilibrium with the vacuum just ahead of the throttle plate.
So, the thicker the oil, the slower the piston rises (to the same level
is would go regardless of what oil is in it) and thus the richer the
mixture gets (or the longer it stays richened).
Brian '69 2000
Tampa, FL
web.Tampabay.rr.com/oilleak
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