The venturi size for the 248 would depend on the actual carburetor number, but probably 30~31mm.
Please understand that not all 228 carbs have a 29mm venturi; nor do all 270 carbs have a 33mm venturi. I simply picked the two carbs used in 1952 as an illustration.
A GENERAL RULE OF THUMB (and when I was in the military, I used to tell the SGT that "general" rules were made to be broken by us "privates"):
When migrating an O.E. (original equipment) carburetor from one engine to another; BEST RESULTS will be obtained if the donee engine is within plus or minus 3 percent of the displacement of the donor engine.
Thus 3 percent of 228 is roughly 7; so a 228 carb could generally be used successfully on an engine from 221 to 235 CID.
Using the 248 example: 3 percent of 248 is about 7 1/2 (round to 7); thus the 248 carb could probably be used successfully on engines from 241 to 255 CID.
The engines need to be of similiar design and RPM for this rule of thumb to work.
Also generally:
Placing too large a carb on an engine will cause LEAN running (due to decreased air velocity in the venturi area).
Placing too small a carb on an engine will cause RICH running up to the point of where the carburetor cannot supply the required air for the engine.
For best results, always adhere to the "5 P's" of engineering!
Jon.
