Take off that %$&*# air cleaner and your plugs won't foul so much. Imagine yourself running down the street with a handkerchief covering your nose and mouth. Can you breathe well enough to reach your maximum speed? Same thing with the Model A carburetor. It was not designed to have an air filter stuck on it, especially the restrictive Air-Maze style that you have. Clever air cleaners have been adapted to the Model A by modern owners that will allow good breathing, but they are bulky and unsightly in my opinion.
I see that you live in Houston, which is probably rather dusty. Maybe the Air-Maze helps filter dust in the air in your neck of the woods, but it definitely will make your engine and spark plugs soot up. Try running the engine without the air cleaner as a test and see if that helps reduce sooting and improves performance. Remember: those 5 million Model A's built in four years did not have air cleaners and they seemed to have longevity on their resumes. Unless you live in a very windy and dusty desert-like area, you don't need an air cleaner. Performance and gas mileage will suffer if you use one.
If you absolutely, positively must run an air cleaner, however, you need to balance the atmospheric pressure between the two chambers in the Zenith by connecting them with a tube. Zenith realized this design flaw in their after-market Model A carbs and put a built-in passage between the chambers in about 1936 or 1937. You can achieve the same thing by externally connecting the chambers. Still, the filter element in the Air-Maze will be too restrictive. Try to find a friendlier filter. Maybe K & N has something that can be adapted? Their filters allow good breathing, but also supply excellent dust filtering capabilities.
Marshall
