differences between 7040273 & 7040263

[unruhjonny]

I have some MPC's so I can identify primary jet, or metering rod differences, but was wondering what really differs between the two assemblies;
I know the primary butterflys also have the little hole ~ of which I can get from one of my spare Oldsmobile carbs (iirc pt #7043250; the curious thing is that I have two of these, but only one has the primary butterfies with the holes!?)

I realize that each carb needs to be tweaked to run for each engine, and I have been collecting parts for the past ten plus years - but I don't know if anything on the main body or baseplate needs to be adjusted...

Basically, my hairbrained logic is that if the '273 was the top dog '70 Pontiac Rochester, that it has to have improvements over the 263...

I appreciate any insight that can be shared.

[Ethan1]

Welcome to CHP!
 
They used different sized primary jets and primary metering rods, but I am sure you knew that. As for other differences, not sure. Someone will know for sure.

[qjetsrule]

the difference may not even be in the jetting, something like a different power piston spring or secondary rod hanger, maybe even a special vacuum port for the ram air or linkage distinguishes a 63 from a 73. remember they were slapping thousands of these carbs on a day, so they had to be perfect to go for the application. I would've loved to be there in the engine assembly plant at Pontiac.

[Cliff Ruggles]

The differences go far deeper than just jets, metering rods and PP spring.  They baseplates and airhorns are quite different, even though the baseplates look the same at a glance.

The airhorns are completely different parts, and this is pretty obvious by looking at them.

I will NOT provide any specific information about the Ram Air carbs beyond that, as WAY too many folks are stamping them out and trying to pass them off as the real deal.  No need to help them out any......Cliff