Idle Air Adjustment Needle Valve

• The Idle Air Adjustment Screw controls the volume of air added and mixed with the fuel introduced through the Idling Jet to the Idle Port. The combination of Idling Jet and Air Adjustment Screw controls the idle air to fuel ratio. Remember, the Idling Jet receives its fuel from the Secondary Well that gets its fuel from the float bowl through the Compensator Jet.
• As the Air Adjustment Screw is turned in, the idling mixture is enriched due to less air being mixed with the idling jet gasoline. (Mixture is leaned down, as it is turned out)
• From factory the head diameter was 3/8" or .375" from 1927 through 1929; and 5/16" or .3125" 1930 through 1931. Through 1929, they were made of brass and thereafterm from steel.
• From factory the shaft lengths were 1 & 3/16" or 1.185".
• When restoring carburetors today I prefer needle shank length of 1 and 1/4 inches, to minimize risk of totally compressing the spring before valve is closed. There are some floating around that are a lot shorter, which causes the spring to collapse and stop before it can seat properly. If Air Adjust Screw Needle is too short or misadjusted, or if upper casting hole does not allow a good seat, the result will be an erratic idle.
• The threads are M5 X .75 (Metric).
• Loose air adjustment threads, in casting, can allow vacuum leaks and result in an irratic idle. This can generally be solved by applying high temperature grease on threads, or if leak is significant a small o-ring can be inserted between casting and spring.
• Off engine initial setting should be 1 & 1/2 turns from full clockwise.

ˆ Top of Page  •  Idle Jet