The power circuit of the YF is both mechanically controlled and vacuum controlled. Unlike the Rochester it lacks the wonky power piston and relies on a simple and effective metering rod of different diameters which rides in the main jet (you can see it at the 12 o'clock position in the last image above). Vacuum pulls the diaphragm downward against a spring's pressure and unlike other carburetors, when you accelerate, the linkage pulls the diaphragm UP instead of forcing it down. In the down position, the thickest part of the metering rod is inside the main jet and so the leanest mixture exists. As speed increases, the linkage mechanically moves the metering rod up to expose the smaller part of it in the jet and when more power is needed (and vacuum falls), the rod is pushed upward by the spring. But if the housing in question is warped, that constant vacuum will suck gas right out of your float bowl all the time and especially on deceleration. You may be thinking (as I did) "There's only a shade over 1 inch between the center holes of those 4 screws, so how warped can it be?" The answer is it can be more warped than you would expect. Let's look at what I found on the housing in image 1 above. In the first image I've set the housing on a known flat piece of metal and you can see there's .006" of clearance at the middle. There's actually more in dead center, but my blade is 1/2 inch wide so that skews things a bit toward the lower side. Three sides suffered warpage of .006" and in the second image below you see there's .008" of clearance. The 3211s I mentioned had a bit more than this and that much space times 4 sides equals trouble. In the third image below you can see a bonnet I removed from a NOS 3211s I own and it also suffered some warpage (not enough to worry about...002 to 003", but it suggests that took place when the carburetor was manufactured). If you look at the 4th image below, you can see where you may also see fuel on the outside of the carburetor body if this is happening. Along the rear gasket between the throttle body and float bowl (also in front by the number stamp) and also down the back side of the throttle body below or around the shaft. You can also see residue on the carb spacer at the rear.
To correct this I got a known flat piece of wood and set a common file on top of it. And then slowly and very carefully I drew the bonnet across the file until all 4 sides were flat and smooth. After I put it back in, the carburetor is running much better now and without any of the richness issues. If you have any questions, please let me know.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
