Does your 1955/56 or E-model 57 idle rough or too fast and appear to have a vacuum leak that you cannot find? The model 4000 carburetor sometimes develops a vacuum leak behind the choke plates were the throttle body meets the main body. When this leak occurs the carburetor usually cannot be idled down much less than 1000 Rpm. The usual cause for this leak is over torque of the two large throttle body screws which distort the main body contact to throttle body. It can sometimes be checked for leakage with the engine running at idle speed by sealing the area behind the choke plates with a thumb and finger. The main body can be checked for fit at the throttle body contact point with the air horn removed.
In most cases the fit between the main body and throttle body can be corrected. In extreme cases two gaskets can be installed between the main body and throttle body to obtain a proper seal.