The 3 screw SU carb allows the coolant to flow through the base of the carb. The 4 screw doesn't. Does anyone know in what years the 3 screw and 4 screw SU carbs were offered? Was the 4 screw installed after the 3 screw ?

i think the 3 screw were 70, then 4 screw were to 1972.

you may check www.ztherapy.com for specific details- i don't recall the details. both my cars have 4 screw, FWIW they are supposedly easier to keep the dashpots moving freely with 4 screws rather than 3. Zya, JA

The 4 screw were used through the '71 model year. The 3 screw were used starting with the '72 model year. I have some of both but I haven't spent any time comparing them.

Have fun!