Gary,
The position of the piston, and thus the needle, is a function of the
vacuum above the piston and the spring. The vacuum is pulling the
piston up; the spring is pushing down. The higher the piston, the more
fuel (the cross sectional area of the needle at the jet gets smaller).
So a stiffer spring will require more vacuum to move the piston a given
amount. So you'll weaken the fuel-air ratio (lean).
"Tuning S.U. Carburetters" by Speed Sports Motorbooks has the following:
blue 2.5 oz
red 4.5 oz
yellow 8.0 oz
green 12. oz
I don't know if the ounces given is a stiffness (oz/inch) or a preload
at a given height (suspect the latter).
The book also states: "A correct strength of the piston spring will be
one which allows the piston to reach it's maximum travel at the point in
the speed range where maximum power is obtained."
The needle profile would then be used to set the fuel-air ratio.
Cheers,
Bob Haskell
AHCA 3000 Mk I registrar
http://www.ciahc.org/registry_3000mk1.php
On 03/19/2013 07:48 PM, warthodson@aol.com wrote:
> Alan,
> I am trying to determine two things as follows:
> 1. I would like to have a basic understanding of what the affect of changing
> carb spring rates has on an engine if no other component is changed. Kind of
> like changing needles, of which I have a basic understanding.
> 2. I have a BJ8 engine with 10:1 compression, a 272 degree Elgin cam and
> triple HS6 carbs. There is no factory spec for this set-up. So I am interested
> in knowing what I might expect if I were to make a change to the springs.
> Gary Hodson
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