So they let in less fuel with a smaller jet and less air by only backing the screw out 1 and half turns instead of 2 full turns.
Probably not. The different settings between the 500 and 550 are far more likely due to the displacement change. A larger displacement engine draws more air volume and, therefore, the depth of the partial vacuum presented to the jets is deeper when using the same carb and air filter type. The partial vacuum is what draws more fuel through them. The reduction in jet size and air screw adjustment (as well as changes to slide needles and emuslion tube holes) is to compensate for the draw characteristics of the engine.
From information I've been able to glean over the years, the carb setup stamping is as follows:
627A - 1971 CB500
627B - 1972 and 1973 CB500
022A - 1974 and 1975 CB550K
087A - 1976 CB550K
069A - 1975 through 1977 CB550F
X46A - 1977 CB550K (K3)
PD 46A - 1978 CB550 (K3)
?? - CB500 (K3)
?? - European model CB550 (K1?) with air restrictor horn.
It should be noted that, except for the last two) these are for American models. I've had discussions with some European members who have machines with an air restrictor on the intake filter box. These have yet another set up stamping on the carbs, with smaller main jets and, I'm fairly confident, other internal parts differences. It's not likely I'll ever encounter, or measure these carbs.
I've compiled a list of known differences and the chart can be found in the FAQ.
I've never actually seen the 627A stamped carbs. But, I've been assured they do exist by forum member's claims of visual verification.
I don't know the internal differences, however.
I currently have examples of the rest, and plan to measure and collect data regarding the internal pieces to note the differences. Unfortunately, several life issues have interceded, and I don't yet have that data, beyond what is currently placed in the FAQ's chart.
Cheers,
