Main Jet

• The Main Jet delivers the major portion of the fuel to the engine at 30 plus MPH.
• When installed with a washer, its tip should be level with the Lower Casting shelf that the Venturi sits on. So, when you remove a Main Jet, make sure you also dig out the old gasket. If you end up with multiple gaskets installed, it will cause poor performance and leaks, because the Jet tip isn't precisely where it should be in the Venturi.
• The Jet needs to have an indented (dished out) tip, to prevent capillary action leaks when engine is off.
• Original Jets were marked with a number to signify orifice size. From Oct 1927 through June 1928 they were numbered "20", which was 0.0394" diameter. From July 1928 through 1929 they were numbered "19.5", which was .0384" diameter. From 1930 through 1931 they went back to a number "20".
• To resize jet orifice: Heat jet at orifice end and fill with solder (Do not over heat or fill beyond 1/8" deep with solder). Carefully remove solder from indented area with a 5/64" drill, using a pen vise (not a drill motor). Carefully drill out orifice (With pen vise! Do not use drill motor) and clean out with air pressure.
• The issue is that the best flow should be 150 to 160 milliliters per minute, at the atmospheric pressure that your driving at.
• However, a little better engine performance, at the sacrifice of miles per gallon, can be accomplished by drilling the main jet with a #61 or #60 drill (170 to 185 milliliters per minute).
• I definitely recommend flow testing of jets, particularly those that have been soldered and drilled.
• Very good instructions for doing jet flow testing can be found in "Zenith Model 'A' Carburetor Restoration Guidelines".
• The Main Jet is usually the cause of high-speed carburetor problems.
• If the orifice is too large, the excessive flow will result in: A rich mixture, low mileage, sooty plugs, smell of gas and irregular running.
• If the orifice is too small or restricted, the insufficient flow will result in: A lean mixture, reduced high speed, and it might even cause backfire at high speed.
• Bullet shaped tips are notorious for causing low flow leaks when vehicle is parked.
• The threads are M5 x .75 (Metric)

ˆ Top of Page  •  Gaskets  •  Main Jet or Cap Jet Tip is not "dished" / "indented".  •  Main Jet Orifice contains dirt or rust, or is too small.  •  Gasket at Main Jet is too thick or multiples are used, resulting in tip being too low.  •  Jet Flow Testing Setup  •  Small Drill Sets  •  "Zenith Model 'A' Carburetor Restoration Guidelines"