I had a similar issue back in the mid-1960's with a flathead V8 in a 47 Ford coupe. The original Holley 2-barrel for that engine had 15/16" venturis, and I found a carb for a 1955 Ford 292 engine that had 1 1/16" barrels. Since I knew my engine had been bored 80 over by a former owner, I thought the 252 cubic inch engine (up from 239) could handle the extra carburetion. Wrong! at moderate speeds in high gear, I had to modulate the gas pedal very carefully to avoid a flat spot. I could slap it into second gear and get the RPM up, and it would run like a banshee! Ditto for top end, but the midrange in top gear was just not there!

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