KLAS - 26/7/2016 2:13 PM
Burlen Fuel says:
Piston springs
On 1 1/4 in., 1 1/2 in. and 1 3/4 in. diameter horizontal carburetters, the red
(4t ounce
) spring is normally used for initial testing and in most installations this load of spring will be effective, assuming that the size of carburetter has been correctly chosen. When the correct spring is fitted it is usual to obtain full piston lift at full throttle and at approximately three-quarters of the maximum rev/min. If the absolute maximum of power is required, however, it is more usual to choose a somewhat larger carburetter, which will not obtain full piston lift until nearer maximum rev/min.
http://sucarb.co.uk/technical-su-carburetters
i would have said, at peak torque, but i haven't touched a carb since years