Hi Tony,
Oldmics has a good eye!
- The rear carb is not the Holley 4000 Ford used in 1957 for the 'E' code set-ups. The presence of the spark control valve is the 'tell' on that. With the choke housing attached to the carburetor body, I'd expect it to be 1956 vintage.
Unless the rear carburetor has been modified there are two immediate concerns.
First, assuming that it was originally purposed as a '56 single quad application carburetor, it has no balance tube to allow connecting it to the front carburetor to equalize the vacuum signal to assure that the secondaries on both carburetors open simultaneously.
Second, the rear carburetor is the source for the vacuum signal provided to the distributor vacuum advance chamber. The '56 Holley 4000 distributor vacuum signal source for the distributor is not correct for the '57 distributor. The Holley 4000s used on the '57 'E' code cars had a different distributor vacuum take-off point and the elimination of the spark control valve on the '57 'E' code carburetors verifies that.
There are other 'changes' that should be made if you want the best performance from this carb IF it is partnered with a '57 'E' code carb on the intake manifold OR IF it is partnered with another '56 vintage Holley 4000.
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-After the end of the '57 model year, Ford continued to supply parts for the dual quad set-up. Example, there were valley pans produced to sell over the Ford parts counter to work with the '57 'E' code intake manifold. The later produced valley pans 'worked', but were slightly different because the other valley pans being made for y-blocks at that time had a dome at the base of the oil fill tube. So, the later 'E' code replacement valley pans had the dome, too. To the point, I've been told by someone more knowledgeable than me that the center bell crank on your engine is likely a later version. Made by or for Ford, but 'slightly different'.
Again, told to me, the throttle linkage rod between the firewall bell crank and the bell crank between the two carburetors is probably adjusted the way it is, i.e. it appears to be extended excessively where it connects to the firewall bell crank, because of an inaccuracy in the making of the later replacement center bell crank. The suggestion is that the two bell cranks on your engine do not align per 1957 original equipment pieces. Your firewall bell crank appears to be correct, OR it's a faithful reproduction.
Does your 'Bird have an 'E' data plate or was your vehicle retrofitted with the dual quad set-up?
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One addition to what Oldmics mentioned about the intake manifold clamps. The rubber hose going to the vacuum advance unit on the tach drive distributor is a retrofit. The factory installation used a steel tube between the rear carburetor and the distributor. If you're 'efforting' to make it look 'original', reproductions are available.
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Oldmics request for additional pictures of the entire set-up would be helpful in advising you further. -Especially in light of one carburetor definitely not being original.
Hope this helps.
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York