Back in my younger, dumber days, I learned the "too big carburetor" lesson the hard way. My 1947 Ford coupe had a flathead V-8 engine that I believed to be a 239 cubic inch 100 HP model. Later, I tore it down and discovered an .080" overbore, making it actually a 252 cubic inch engine. It had the standard Ford/Holley 2 barrel carb with a 15/16" venturi diameter. I found a 2-BBL carb for a 1955 Ford 272 engine that would bolt on with minimum modifications, mostly tinkering with the throttle linkage. This carb had a 1 1/16" venturi size, not that much bigger- - - -right?
The engine never ran right with that carb, even after a rebuild, and bigger main jets! It had a horrendous flat spot off-idle that could only be avoided by gently opening the throttle to maintain enough venturi velocity to keep pulling fuel through the jets. Even a slight bit too much throttle opening killed the venturi velocity, and the engine went too lean to accelerate. That was usually followed by a series of backfires that would blow the air cleaner loose from the top of the carb. Putting the original carburetor back on it solved the problem. At highway speed, the engine ran fairly well, but I lost everything but full-throttle performance. Not an acceptable trade-off!
Jerry
