[55_327]

You must be getting at least some fuel flow through the secondary jets or you'd have a massive bog when the secondary throttle blades are wide open, and you'd actually run faster on just the primaries. I'd soak the carb body and metering block in carb cleaner and blow out all passages for starters. Then again, maybe that Chrysler metering block is somehow not compatible with a standard 4778 carb.

On another note, has the engine run as expected in the past with another carb?

 

[55_327]

There is a third circuit or something in that block, I'm not familiar with what it is but I can definitely tell you just by looking at the cast shape that the block is incorrect. It likely has altered and/or additional air bleeds and passages and their locations....so passages may not even align with the main body even though it bolts up..
It's been so long since I looked at one, it may even be a reverse idle metering block.

Also, best I can tell it is a primary metering block, and not a secondary.

You'd be $50 farther ahead to just order the correct replacement metering block for that 4778.

Great catch, Eagle Eye.