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The Place for Model-A Ford Carburetors

Cap Jet

Cap Jet
  • The cap jet delivers most of the fuel at low speed driving.
  • It gets its gasoline from the Compensator Jet and the gas Adjusting Valve.
  • It is removed from the lower casting with a 5/16" nut driver
  • When installed with a gasket washer, it's tip should be level with (to 1/16" below) the lower casting shelf that the Venturi sits on.
  • Jet needs to have an indented (dished out) tip.
  • Original jets were marked with a number to signify orifice size. From Oct 1927 through June 1928 they were numbered "19", which was 0.0374" diameter. From July 1928 through 1929 they were numbered "21", which was 0.413" diameter. From 1930 through 1931 they were numbered "20", which was 0.0394" diameter.
  • I recommend that orifice is drilled with a #62 drill. (That's 38 thousands of an 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 in the range of 170 to 190 milliliters per minute, but is not critical.
  • If the jet is too small or restricted, the insufficient flow will cause a lean mixture and miss or jerk on hard pulls at low speed.
  • The threads are M5 x .75 (Metric)
  • Occassonally, the cap jet is broken off due to over tightening. It is best to remove a broken cap jet by starting with a small drill and then sequentially using larger drills to slowly drill away the barrel of the jet. Be careful not to destroy the castings threads and clean the brass out of the threads with a M5 x .75 (Metric) tap. Sometimes brass debri may need to be picked out of the casting.

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