Here is the factory idle bypass air entry. This is a 9103 carb. 1977 400 AT HP.
View attachment 1715871287 Air then passes through the main body and out of 2 holes in the base plate. Here my .099 drill bit fits snug.
View attachment 1715871288 On the top side I drill and tap for a 10/32 x 1/8 brass set screw. It’s a #21 drill.
View attachment 1715871295The factory.100 hole will accommodate a pretty good cam. You will sometimes notice that these later carbs will have the mixture screws out 4 or more turns or clear out of adjustment and still are too lean at idle. Then you have to enlarge the idle feed restrictors (IFR) and may even have to drill and tap the idle air bleed to reduce its size and richer the idle mixture further. On a pretty nasty 451, I’ve the idle bypass @ .128 and the IFR’s @ .052. It is a balancing act and takes a little patience to get it perfect.
I should also mention, set your transfer slots with the proper exposure before you start testing and resist temptation to compromise that setting throughout the tune.
