Jim---SU carbs come with three jet sizes--.090, .100 and .125. The
needles are matched at the first station to the jet size of that
particular carb. The taper is then machined according to engine needs.
Using an air/fuel monitor, I have tried various springs, with no
surprises. The light springs and shorter stock springs allow the air
valve to rise slightly faster, as expected. The rate of the spring,
along with steady throttle position, sets the air valve height. (Lighter
springs, slightly more mid-range power, heavy springs, better steady
state fuel mileage via leaner mix.) The viscosity of the damping oil has
the most effect on the speed in which the air valve rises during
acceleration. It's difficult to get it better than what the engineers
came up with for the stock engine. I believe the same to be true with
the ZS carbs.
Dick
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