I've been working on my Autolite 4100 carb and have a question about the secondary venturi check ball. First off, my car was built in February 1966. The indented code on the front drivers side of my carb is "6Z B", being a 66 Mustang, design change B I believe.  Under the bottom of the carb, on the drivers side secondary, there is a cast-in code of "6-65". I believe that this could mean June 1965. If this means the design change date, then it makes sense give the build date of my car and the fact that I know it is the original engine and carb. Under the bottom of the primary fuel bowl there is a 1/2 inch high number "7". I have no idea what this stands for.

When I took it apart there was no check ball under the secondary  venturi cluster. All the carb-rebuild articles I've read say that there is a larger secondary check ball under the secondary venturi than there is under the primary venturi.  What puzzled me after not finding a secondary check ball was that the so called small(er) check ball doesn't even fit in the secondary venturi hole. I've been driving the car for the last two years and the response has been flawless as expected. The check balls in the 4100 rebuild kit I purchased didn't fit the secondary venturi hole either. They were all too large.

The primary check ball covers a hole that leads from the venturi hole to the accelerator pump on the drivers side front of the carb.  I verified this by finding a humped casting in the bottom of the primary bowl and by putting air into the primary venturi hole and watching the accelerator arm move back and fort when I released the air. I got to thinking that if the ball covers a hole in the bottom of the primary venturi then the secondary should be similar, right? But when I tested the secondary venturi hole this same way there was no air coming out of any hole or moving anything. In short, it appears that there is no reason to have a secondary venturi check ball in my 4100 carb.  I could not find any casting evidence to suggest a a passageway under or in the secondary bowl either.

Putting it simply, it appears that my 66 Mustang design change "B" 4100 carb had no secondary venturi check ball to begin with. Has anyone found the same thing with a 4100 carb? Does anyone know if there was a 65 design (maybe design change "A" and "B") of this carb that didn't include a secondary venturi check ball?  

Ok Scott let me see if I can clear it up because I think you have some misunderstandings there. First off there is no secondary venturi accelerator pump and so no check ball in any 4100 there(that I am aware of). The check ball in the primary side does two things, it prevents backflow when the pump discharges and fuel pullover at high speed. Thats why you find a weight in many of them. That weight holds down the check ball. In the secondary, there is no need for an accelerator pump shot because you're already drawing a lot of fuel and a pump shot would be redundant so there is no passage in the seconday (booster)venturi for that. Where you DO see a check ball in the secondary is in the vacuum passage inside the secondary operating diaphragm controlling the rate at which they open. You can access this thru the secondary diaphragm plate on the back of the carb.
That carb really hasnt seen a lot of changes but just in case I checked 1958 manuals and 1966 manuals ,,, they were the same.  

With the absence of an accelerator pump for the secondary, as you say it's not needed, everything you said make complete scene. I better go back and read the Mustang & Ford, May 1997 article, Weekend Projects, Carb-Rebuild Tips, Understanding the Autolite 4100 and 2100, by Jerry Heasley. It's great that we have this form to ask questions and get answers. Even more, it's terrific that there are people like you that are willing to provide your experience and expertise to people like me. Thanks very much for your help.  

Your welcome Scott for sure. The problem I have with a lot of these articles is that they are written by mechanics or spec writers. Most are very knowledgeable about the carbs but they dont always know or tell the reason for some of these things. People like you need to know the reason behind something and then it makes sense to you. That's why in most of my postings I try to explain the reasoning the engineers had in doing what they did. My expertise isnt always perfect in some areas because I have forgotten more then I care to acknowledge,,and I dont have access to those fuel systems guys at Ford anymore,,,they were the best.  By the way I think the check ball for the secondary is behind that little screw above the diaphragm plate and not inside.