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The Place for Model-A Ford Carburetors

Idle Jet

Idle Jet
  • The Idling Jet provides gasoline (from the Secondary Well) to the engine at idle.
  • Original Jets were marked with a number to signify orifice size. From Oct 1927 through June 1928 they were numbered "10", which was 0.0197" diameter. From July 1928 through 1931 they were numbered "11", which was 0.0217" diameter.
  • I recommend that orifice is drilled with a #74 to #75 Drill. (That's 0.0225 - 0.0210 inch).
  • 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 flow should be between 44 and 48 milliliters per minute.
  • I particularly recommend flow testing those that have been soldered and drilled.
  • This is a very small orifice hole and flow. It's easily clogged by dirt, rust, etc.
  • If the jet is too large, the Air Adjustment Screw will need to be turned out very far.
  • If the jet is too small or restricted, it will be impossible to obtain a satisfactory mixture, except by turning the Air adjustment screw all the way in.
  • Some carburetors, on some engines, need to have the Idle Jet slightly larger or smaller than a #74 Drill.
  • No washer is used with this jet because it is a taper fit.
  • Watch for fractures / breaks along the shaft and stems that turn in its base.
  • The total length of an Idle Jet is 3 inches.
  • If you run across one that is longer than 3 inches, it's a little rare, and it belongs to a Zenith Carburetor with a Double Venturi. The Double Venturi carburetor did not originally use a Brass inserted secondary well, so to compensate, the Idling Jet was made a little longer. Less than 5% of the carburetors made were Double Venturi.
  • The threads on the idling jet are M5 x .75 (Metric)

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