i have a friend who has a weber carb that will sell to me for cheap but its a 40mm and i was looking around with search and didnt find specifcs on 40mm to 48mm is it just ventureis or more i have never torn apart a weber so any expierience on this would be great.

 

You can fit different size venturis in either one. The 40 or 48 refers to the diameter of the top stack. You can't fit 40mm venturis in a 40 mm carb(there would be no narrowing of the air pipe for venturi action) A street car could get away with a 40mm carb, I don't know what the biggest venturi available for them is but it's what you would want. The 48 would be better, with maybe 40mm venturis.

 

then a 40mm carb wouldnt be most suitable on a stock port 13 6 port.

 

nah, get a 45.