Jets and Passageways
From OpenContent Curriculum
A word about these carburetors: They aren't as complicated as they may appear. The carburetors are made so that their external adjustments may be accessed from either the left or the right side; (not both on the same example.) Because of this they have certain features which are cast into both sides, but completed only on one side. This includes provisions for the idle screw, pilot air screw, slide indexing pin, passageways, etc. Don't be fooled or confused by this. Some of the holes are empty dead ends. Further, brass plugs cap the ends of passageways that were drilled after the casting was made. Sometimes more than one passageway hole is drilled in order to complete a passageway which makes one or more turns. These plugs are not a maintenance item and should not be disturbed. The only way to access or clean the passages is by using compressed air or aerosols.
The Carb Throat
This is a view through the carburetor throat, looking from the engine side of things - outward. The flange of the needle jet is visible in the lower section of the throat. To the left, the rounded nose of the idle screw screw is visible.
The air intake side of the carburetor throat; air intake ports can be found in three of the holes.
- The lower center hole feeds the needle jet.
- The ports to the left and right and slightly above it are feed holes for the air screw. On this carb the air screw is on the right, so the lefthand hole was cast but not drilled out.
The two smaller ports next to the needle jet port exist in order to drill passageways. The right-hand one was drilled as a passageway for the air pilot screw and then capped with the brass plug.
- On the righthand side, slightly above the three-o'clock position, the hole feeds the fuel enriching starting circuit.
The air ports and passageways must be open. Remove the screws, jets or other parts to inspect and clean them properly. Shine a light into them for inspection. Use compressed air to clear them. (Protect your eyes.) The images below give some idea of what you will see.
Inspecting the Air Intake Ports
The needle jet port can be viewed (from either direction) when the needle jet is removed.
By removing the air screw (and spring), and shining a light into the hole, the port can be viewed. (The passageway makes a 90º turn past the screw and follows a smaller passageway drilled into the carb body.)
With the enrichment -often erroneously called the "choke"- plunger removed, a light can be shined into the plunger seat area. Light should be visible in the port.
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