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Is the size stamped on the venturi somewhere?
How would I measure the size?

This a 1965 carb if the matters.
The venturi size is stamped on left side of the front float bowl. If your carb is original it should read 1.08. If it reads 1.12 you either have a HiPo carb with a manual choke or someone has replaced it with an improper carb.

If it is an original, do not let loose of it. There is no better carb for the small block Ford. They are the simplest, most reliable and most economical 4 bbls ever built.

Once you determine the venturi size, you may want to pull the top, check the primary and secondary jet sizes and the two letter codes stamped on the primary and seconday annular boosters. This will confirm that it is the correct carb. All 1.08" 4100's were not created equal.
 

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Are the annular boosters the same as the venturi?

What letters am I looking for on the annular booster?
The annular boosters are the rings that hang out over the upper entrance to the venturis. These are the magic of the 4100. Instead of squirting a stream of fuel down the venturi, they spray out an atomized mist from around the ring.

They are held in place by one big screw, once you take the top off the carb. I think it is the secondary booster that has a hollow hold down screw that contains a spring and a checkball, so be careful when you remove it.

The one or two letter part number is stamped on the underside of the booster base. There is a little gasket that may come out attached to the booster mounting base. You would have to remove it to read the letters. A C5ZF-D carb would have had a CH primary booster and an A secondary booster. The primary jets should be 48F and the secondaries, 58F

One thing I forgot to tell you is to look for the actual carb partnumber that is stamped on the side of the left front foot of the carb. It is right below where the venturi size is noted on the float bowl.

A '65 automatic would have been a C5ZF-D or F, a 4 spd would have been a C5ZF-C or E.
 

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Are the annular boosters the same as the venturi?

What letters am I looking for on the annular booster?
Get a '65 shop manual. The specs at the end of Section 10 list the details of what Booster Venturis were used in what models as well as the throttle bores, venturi and jet diameters. There are probably at least a half dozen variations. If you don't have the correct set of numbers - good luck on finding them. Fortunately, they are not drastically different so using the 'wrong' one usually isn't a big deal.
 
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