As Fetch says, it may not be the carb at all. I can go over a few areas to check but need the history first before trying a few tests and firing off a bunch of guesses gets nowhere. So..........
1. Starting with the first rebuild, did you rebuild because it was doing what you say?
2. Was it ever running good? When? How long has this been going on? weeks months years?
3. Did it do what you say each time after each rebuild?
4. Give us an idea of what it did at the beginning and what is does now.
5. Have you done any other work to motor or wiring or timing or ignition of any kind along the way? Replaced any other parts?
6. Do you have a 305A or other? B C D E
7. A 1 bbl Stromberg is not listed on our carburetor page for your motor....or Stromberd
Need careful and complete answers please.