American Motors: 1959 Ambassador V8 4 barrel, vacuum accessories, compression check

Question
My engine needs nearly 3/4 choke to run well. Engine races up well when you hit pedal at first but then breaks up if pedal is held steady. I have disconnected all vacuum accessories with no improvement. Good vacuum at idle (17)

Tried good carburetor from another car, the same problem. Sprayed choke cleaner around intake manifold, when I sprayed around exhaust crossover passages at center of manifold, idle picked up !

All tune up settings at factory guide.  Timing advances like normal.

I then removed intake manifold and could not see any cracks.
Compression check gave 140+ to 155. except second from rear on passenger side, it was 115.

Do you have an idea where to look...THANK YOU

Answer
When you reinstalled the intake manifold, did you use sealer around the manifold gasket and properly torque the bolts?
Check the craburetor to manifold gasket(s), too.
If the manifold is still off, use a straight edge to check for manifold warpage.
The 115 cylinder should be checked out. It shouldn't have anything to do with the concern you described, though.