The size of the venturis (the restrictions about halfway down the body of the carburetor) must match the size of the engine to allow the fuel and air to mix properly. A too large opening will let too much air in, and won't mix the right amount of fuel with it. That's known as running "lean". The driver opens the throttle further, which makes the problem worse because the engine gets even more air. Eventually, the "power" system of the carb opens up due to low intake vacuum, and the mixture gets VERY rich. If a carb with the proper size venturi is used, it requires less throttle opening to get the right mixture, and the power system only works while passing, hill climbing, trailer towing, etc.
Virtually all 250 engines were equipped with 1 barrel Rochester carburetors at the factory. A few Mercruiser marine 250s ran a small 2 barrel Rochester carb. Someone has installed an aftermarket 4 barrel intake and adapted a clone of a Ford carburetor to the manifold that's totally inappropriate for anything other than racing. All the adjusting screws do is allow the idle mixture to be adjusted for the smoothest operation. Once the engine gets up to 1500 RPM or so, the idle screws have no effect at all.
The Ebay link I posted is for a Ford 260 cubic inch V8, which has only slightly more cubic inches than a 250 Chevy. One with that calibration will probably be closer to the right performance without having to swap the intake manifold back to an original 1 barrel intake.
Jerry
