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The Place for Model-A Ford Carburetors

Filtering the air going into a Model A carburetor is a controversial subject among Model A mechanics. This information is provided to help people make their own decision.

Filtering Air

K&N Air Filter on a Carburetor
  • Carburetors, with their air input totally unfiltered, suck in all kinds of stuff that can do damage to your engine. Fine silica dust and powdered rock are the most common materials that are pulled in and cause engine wear.
  • The Air Maze, metal gauze unit (sold by most Model A Ford part distributors; shown on the left) prevents large particles from entering, but does nothing to filter out the small stuff that most commonly causes the damage.
  • The next logical step is to try paper element cartridges. However, you have to be careful not to restrict the air to fuel balance of the carburetor. The level of air resistances varies depending on the size, surface area, and the physical attributes of the filtering material.
  • An undersized filter restricts airflow into the carburetor and lowers the pressure at the choke. These consequences make the fuel-air ratio higher (too much gasoline for the airflow reduction) and also lower the velocity of fuel flow through the Venturi (reducing engine responsiveness to throttling). The result is carbon deposits in your engine from the rich mixture.
  • One needs to consider the air volume needs for a nominal 200 cubic inch engine at 2,800 RPM and the carburetors air intake area of 1.767 square inches. You also have to consider how much to oversize the filter element to allow for clogging up to say 30%. With some rough calculations, you'll find that you need a large filter.
  • I've read material from multiple sources, which say: "automotive engineers regularly size air filter surface areas at one-half an engines cubic inch capacity", which is 100 square inches for the nominal Model A engine.
  • I have found paper filters clog up quickly, restrict flow, and retain gasoline from drippy carburetors for a long time. So, I prefer K&N washable, oilable and reusable filters. Using published K&N formulas, I calculated the filter required for a typical Model A engine. If your engine has been modified to increase the cubic inches, you will want to adapt my following calculations.
    • First, I calculated the Effective Filtering Area for the physical attributes of K&N filter materials:
      • 200 Cubic inches displacement times 2,800 RPM divided by 20,839, which is the factor for K&N Filter Attributes = 26.9 square inch Effective Filtering Area.
    • Then, I calculated the minimum length for a filter to fit in an Air Maze housing:
      • 26.9 square inch Effective Filtering Area divided by (3 & 1/2 inch Air Maze diameter times 3.14 Pi) plus .75 inches to compensate for the rubber seals on each end of the element = 3.2 inches minimum filter length.
    • This is significantly longer than the very small air filters commonly seen on many Model A Fords.
    • K&N offers several filters that can be used with the Air Maze housing.
      • Part # E-3050 is 3.5 inches long.
      • Part # E-9257 is 5.5 inches long.
      • Part # E-2040 is 6.625 inches long.
    • To provide some margin for the filter getting dirty, before you get around to cleaning it, I would recommend the 5.5 inch long length. I know it is an over kill, but I am currently using the 6.625 inch long one to allow for driving on dusty gravel roads and to extend my time between filter cleaning / re-oiling cycles.
  • CAUTION: Regardless of any filter that you choose to use, you have to be watchful for a leaky carburetor that can load the filter up with gasoline and be vulnerable to ignition from the exhaust pipe and possibly cause a disastrous fire.
  • Important notes on the use of Air Maze housings:
    • When mounting an Air Maze housing onto a Zenith, Model A carburetor, it is important that a lock washer and nut be used to keep the screw from coming loose and allowing the Air Maze housing to fall to the ground. The screw and nut can be seen at the top of each photo.
    • To minimize air leaks at the point where an Air Maze connects to the Zenith, Model A carburetor, I use an o-ring (1-7/8" OD and 1-1/2" ID) on the carburetors air intake.
    • To use a long filter in the Air Maze, the large wing bolt needs to be replaced with a long 1/4 - 20 bolt or with a threaded rod, with nuts and lock washers.

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