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· More for Less Racer
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Back in the day (late 70s) the 4778 and 4780 Holleys were dogs and very unpopular. Maybe EricNova can address this?
Not that I ever knew of, they just seem overlooked by many so not as many sold, but there weren't any problems with them. 700 and 800 get overlooked because not all companies offer those size would be my guess.
I used an original 4780-AAS, the original roller link secondary design and it was a real good carb, then sold it to a friend and he raced with it for years.
I recall an 830cfm Holley that was unpopular, it was basically the bigger 850 main body with an smaller 800 baseplate to make it 830 cfm. It seemed to have trouble with metering clean below 3500 rpm

for the Op's problem,
I'm going to go out o a limb and say there is something not compatible with that Mopar metering block.....If the picture I just googled and looked at in the Ebay listing for that exact metering block is correct (guy has the original Mopar box too, so looks legit), ...then it is the source of your problem.
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 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.

Packing materials Shipping box Gas Packaging and labeling Box

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