Hey Frank, still trying to figure out what you have?
I was building a 7.5 carb today and thought I’d compare it to a 5.5 and 3 horse carb. All three carb throats measure .900 – .915 at the choke end but that’s where the similarities end. At the butterfly valve end, the 7.5 measures .910, the 5.5 .750, and the three horse comes in at .625. In other words, 29/32nds, 3/4 and 5/8 of an inch. I couldn’t get in to measure the venturis.
Also, a 5.5 carb will be drilled across the top in two places to pick up the throttle linkage shaft. The 7.5 carb isn’t drilled.