ive run carbs on 3 different 3.0l motors, the 1st two were the same motor, but post rebuild I went from weber 40's to PMO 46's. in both instances it was equipped with the stock CIS cams due to race class rules. the spec911 class requires stock pistons, although you are allowed to use the euro higher compression units and stock cams. those are the rules, were they followed by every competitor? not likely IMO.
fuel economy will plummet with carbs. my current 3.ol has hotter cams, 9.5/1 JE pistons and run a set of weber 40's and it's a great set up as far as im concerned. im seeing around 12-13 miles a gallon even at freeway running. less when the drive is more spirited. you are on the right track with the recurve of the dizzy. I plopped a set of 40 webers on a 90k mile early 3.ol and went racing and the motor did 225 at the wheels on a dyno at one point. plenty of power.
im not sure of the cold weather applications, but that could be the fly in the ointment. remember that the early carbed cars in Europe were likely run year around and it's cold and or snowy there, so there's that. when you ask about the jetting and expenses to get them tuned properly and buying a used set of webers you might be better off looking for a set of new PMOs (or lightly used). you can never tell just how good of shape a used set of what could be 50 y/o carbs so there is value in getting new equipment and new PMOs are somewhat tuned to your needs from the outset per your application needs when assembled by PMO. I reacquired the weber 40's I have now that were originally on my spec car prior to the PMO so they were a know entity. you could pay $2500 or more for a set of webers that could be tired, throttle shafts, needing rebuild and be in what a new set of PMO's would have costed when all said and done.
check out the performance oriented website. member 1quicks in the weber guru around here and his site is must read if you are considering carbs.
did I mention you fuel mileage will suffer, significantly?
