As previously stated, I really like the YF design. The only thing I don't like about it is the pump diaphragm (leather accelerator pumps are much more trouble-free).
The key to using a YF carburetor is to use the RIGHT YF carburetor. Carter made literally hundreds of different versions of the YF. Too large a carburetor will run rich at idle, lean at cruise, and be sluggish at low RPM. Too small a carburetor will have a power drop, and cause the engine to run hot.
The link provided above shows a Carter spec sheet from the Carter Master Parts book for a YF intended for use on a Willys 4 cylinder 134 cubic inch engine (not suitable for Chevrolet 6); and the YF repair instructions (these are generic) and suitable for all YF to be used in conjunction with the appropriate spec sheet .
One other thing to consider: Carter used "velumoid" gaskets in the older YF units. These gaskets shrink over time. If you find a NOS YF, rebuild it with a modern kit prior to use. Be very cautious of using NOS repair kits. Check the gasket fit.
As to the quoted CFM of 250? Carter did not publish CFM ratings on any of the early YF carburetors. Some may reach this figure, many will not. Again, it depends on the individual carburetor.
Jon.
