I've been tinkering on carburetors since the early 1960's, first around the family shop, then as a technician and independent shop owner. I've done quite a bit of racing, with gas and alcohol-fueled engines, and I've developed some pretty firm opinions along the way. Some of them might even be correct!
Most people put WAY too much carburetor on their engines, then try to deal with stumbles, backfires, and a host of other problems that can be traced directly to having a too-slow velocity through the carb venturi(s), resulting in lots of air, and virtually no fuel flow. The venturi effect speeds up the airflow through the smallesrt diameter of the carb body (the venturi) and lower the pressure enough to allow atmospheric pressure to force fuel through the jets and into the airstream. Slow the venturi velocity down by putting too big a venturi (or too many of them) in the airstream, and you get LEAN in a hurry. The only fix is lots more cubic inch displacement, or lots less carburetor.
I'm planning to use the Rochester 2-barrel that came stock on the 265 and 283 engines, not the bigger-bore model that was used on the 289 Pontiac and others. The 250 will have a major bump in compression by using 307 V-8 flattop pistons, an .050" head milling job, and a pretty radical hydraulic cam. I might go a little bigger on valve size, too. It will idle at around 1200 RPM, and have a top speed of 6,000 RPM or so. That works out to just over 450 CFM, which the 2-BBL should supply with no problems, and I don't have to mess with multiple carburetors!
Jerry
