The flat tops have an extra valve in front of the damper piston. I think it runs the choke, but I'm not sure. There are some other differences, but for me I think the domes are just easier to work on based on the complicated diagrams I've seen of the flats. From what I understand, my 74 should have flats but when I bought it it had domes, even though the owner said it was all-original.
from what I remember the boat anchors (flats)where extrememly notoriuos for vapor locking...they are also more of a pain to adjust..but mostly due to the vapor locking and they also don't put out as much power as the SU's do....Keep your rounds dont change em!!!!
It has nothing to do with the shape. There are flat-top SUs on 510's that are functionally identical to the dometops and work just fine.
The Z flattops were introduced for one reason only - to comply with more stringent air quality requirements. They have an idle circuit that bypasses the piston, so it can be completely down at idle. They have a power jet, whose function is similar to an accelerator pump, but which is subject to destruction if the car backfires through the carbs.
The idle is set on the balance tube rather than on the carbs and the idle fuel/air mixture is set via a large screw on the front carb only and shared with the back carb via a hose that runs between them. The air fuel mixture isn't set on the carb itself.
The choke doesn't move the fuel jet; it's fixed and cannot move. Instead, the choke moves a second butterfly valve between the air cleaner and the piston, which cuts off some of the air supply to the carb. The float bowl is under the carb rather than next to it, which is why the carb body is so large. The float bowl overflow tube comes up into the space between the carb and the air filter, also, and is suspected to be the cause of the vapor lock problem, which can be avoided by letting the car idle for a few minutes before shutting down, especially if you pop the hood open before shutting down so it can cool off first.
(or should I say apologists), are a very small and elite ("the thinking man's carburetor") bunch! Overrun by hords of roundtop enthusiasts! We will make our stand, like the soldiers planting the flag at Io Jima! One thing in our favor - lots of spare carbs (boat anchors) out there!