[Healeys] Carb piston springs

[E.A. Driver]

The list Bob provided can be used in SU carburettors from 1.25 inches 
through 1.75 inches.  To further quote
"Better engine performance is almost always available if light piston 
springs are used (2.5 or 4.5 oz, for example)
and the needle sized to suit the spring"  and "Many engines have 2.5-4.6 
oz fitted as standard."
Cheers
Ed
Saskatoon, Saskatchewan


On 19/03/2013 6:46 PM, Bob Haskell wrote:
> 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 at 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