> Your idea of measuring vacuum in the piston dome sounds possible.
> Otherwise it is really a guessing game to know what part of the needle
> you are operating on.
>
> I don't know what the vacuum is in the dome, pretty low I would guess,
> maybe zero to six inches.
I hate to be a wet blanket, but it seems to me that the vacuum in the dome
would be fairly constant and also quite low. The underside of the piston is
basically at atmospheric pressure (vents through the face of the carb at
least on the old HS and H series carbs), and the pressure differential is
only enough to overcome the weight of the piston and the force of the
spring.
Ignoring the force on the damper, it takes only an ounce or two to lift the
piston; and if the piston diameter is 3" (guessing here, don't have one
handy) then the area is 7"^2 and the pressure differential to provide 2
ounces of force is only about 0.02 psi or about 0.5 inches of water. This
is probably less than the drop through the air filter (assuming you're
running a filter).
The force and hence the pressure will vary slightly as the spring is
compressed, but the variation is going to be even smaller. At least for my
carbs, the weight of the piston seems to be more than the force of the
spring.
I have sometimes wondered if an optical distance readout, similar to that
used in digital calipers, couldn't be adapted into the dome somehow, but
I've never pursued the idea.
Randall
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