Upgrading my 340

The problem with carb rating it actually doesn't correspond to what your engine uses. It's more a measurement of restrictiveness. Say your 650 pulls 3" of vacuum at full throttle and a 750 pulls 1.5" both carbs would be pulling 750 cfm the rating doesn't matter cause it depends at what vacuum. You can check the amount of vacuum at a full throttle run and that will tell if yours is to small or not.