Difference in a 79' Carb to the other years?

 

The throttle body on 79 carbs has a control for both metering air and metering fuel in the idle circuit. The later ones don't, and are easier to troubleshoot with one less variable. Nothing is gained by using the earlier T-body even in a modded Nikki application (that I have figured out so far for a stock 12A ).
There is a power valve in the 79 (not all) which is prone to blockage by fuel bowl sediment. (25 years is a lot of sediment!) It utilizes a steel spring inside it which easily rusts with water present in the fuel.
If I recall correctly, there is a "richer" jet and solenoid on the 79s in addition to the power valve...Carl knows more than I do about them, having worked on Holleys, ect.
There are only minor differences in jet sizes and return line orifice and AP nozzel orifice size. Generally, they're all slightly larger, but not enough to seek those parts.
The airhorn does'nt have the idle compensation valve circuit cast throughout it like later years, as this was a different design. The later models have the circuit running throughout the wall that seperates the secondaries, and the secondaries from the primaries (looking down through, these are the thin support walls that lead to the center bolt hole). It's not a flow impedence, but without the "lump" in the wall the carb has a cleaner appearance.
The 79s also have a wider primary choke plate, though Carl has flowed the 79 horn to discover the difference is almost worth the effort to chase them down specifically for use on modded Carbs.

Generally speaking, the later carbs are easier to tune, maintain and modify. The 79-80 carbs won't tolerate the same amount of crap inside them.

 

Thanks this info is a lot of help.