Could someone tell me what the difference is between the three screw and the four screw SU carbs in reference to the carb piston housing? Is one better than the other?

I don't believe there is any difference in the carbs internally. One is just as good as the other. Why did they change from 3 to 4? Good question. Maybe no reason other than someone thought 4 screws would prevent it from being warped due to overtightening, I don't think I've ever seen a reason for it in print.

The difference between the two (visually) is in how the "dome" is attached. By either 4 screws (early) or 3 screws (later models).

According to Z Therpy's SU video, the 4 screws are easier to align the piston. Apparently, the 3 screws alow a bit of movement in the positioning of the dome on top.

There are a few other minor differences between the two like the mixture nut is different and little things like that, but the overall operation is the same.

Guess I better get their video. I would have learned something. Glad all of mine are 4 screw, I won't mess up anything. Yeah, right.

I thought I wouldn't mess mine up either, but I still had to buy new choke nozzles because I rushed my disassembly too quickly.

When ya'll work on these things, make sure you are patient and take your time. It'll definitely save you money in the long run.

Actually, I think I'm just going to get Z Therapy's carbs and maybe a polished intake for my 71, that way I know they are done right. Besides, I have a couple sets here I can trade in or practice on later.

After looking at the pics of Marty Rogan's car I doubt if my work would look as good as theirs.:mad: