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VENTURI:

3-bolt venturi is about .870 – .875,

4-bolt is about .780 Diameter.

Disassembly, Cleaning and Repair of Briggs and Stratton Opposed Twin-Cylinder Carburetor

  • (fixed jet, down draft, flo-jet carburetor, no adjustments except idle speed, with integral fuel pump)

    Briggs carb part numbers 693480
    used on Briggs and Stratton Engines 422432, 422435, 422437, 422442, 422445, 422447, 422707, 422777, 42A707, 42A777, 42B707, 42D707, 42D777, 42E707, 42E777.

    • Quick notes: For those carbs that have adjustable idle and/or main fuel mixture screws the preset (preliminary adjustment) for the needle valve is 1-1/2 turns out from lightly seated.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Down-draft carb with fuel pump
  • #1. Down-draft carb with fuel pump (Type „Nikki Six„)
  • ‚Opposed twin-cylinder‘ refers to the fact that the two cylinders are 180° opposite each other, as compared to a V-twin configuration. These engines use a float type down-draft two-piece Flo-jet carburetor with a built-in fuel pump (#1). The float and choke are part of the upper body of the carb. The high speed and idle fuel circuits are separate. Only the idle speed is adjustable. The high and low speed fuel mixtures are set by fixed (non-adjustable) jets.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Pump has 4 mounting screws
  • #2. Pump has 4 mounting screws
  • Four mounting screws are used on later versions of this carb (#2). Earlier versions had three screws. The pump is operated by the pulsations of crankcase vacuum fed into the pulse tube nipple (#2, blue) as the engine runs. Fuel enters the pump at the nipple shown at #2, green. There are two vent holes in the cover (#2, red) that must not be blocked by dirt or insect activity.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Pump cover plate removed
  • #3. Pump cover plate removed
  • Behind the pump cover plate is a gasket and then a diaphragm (#3). Ensure that the diaphragm is not perforated, deformed, brittle or gummy.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Two check valves
  • #4. Two check valves
  • After the gasket and diaphragm is the pump body. Fuel enters the body at the nipple (#4, pink arrow) and flows through the lower check valve (#4, blue). Fuel returns through the upper check valve (#4, red) from the main reservoir on the other side of the pump body and exits to the carb via the passage at #4, green. Check that the check valves are free to move, and free of foreign matter and that they close fully. They are not meant to be disassembled from the body.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Main pump diaphragm
  • #5. Main pump diaphragm
  • The main pump diaphragm (#5) is between the pump body and the carburetor. Ensure that the diaphragm is not perforated, deformed, brittle or gummy.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Spring and cup
  • #6. Spring and cup
  • Behind the main diaphragm there is a spring and a cup (#6) that sits on top of the spring. The cup protects the diaphragm from damage by the spring. Vacuum pulsations from the crankcase arrive at the carb via the pulse tube nipple (#6, yellow). These pulses work against the spring pressure causing the diaphragm to pump back and forth, drawing fuel from the lower check valve and pushing it out the upper one. Fuel then exits the pump body and enters the carb at (#6, pink).
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Pump vent
  • #7. Pump vent
  • There is a small vent at the bottom of the vacuum chamber (#7) that should not be obstructed.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Older 3-screw pump
  • #8. Older 3-screw pump
  • Older carbs used a 3-screw pump and had an adjustable idle mixture screw (#8, green). The preliminary setting for the idle mixture screw is 1-1/2 turns out from lightly seated. There are two vent holes in the pump cover (#8, pink).
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - 3-screw pump has 3 springs
  • #9. 3-screw pump has 3 springs
  • The 3-screw fuel pump has three springs. There are two valve springs that sit on bosses (#9, pink) and one diaphragm spring on which a cup sits (#9, green). The cup protects the diaphragm from damage by the spring. Fuel enters the pump at the side nipple (#9, red) and vacuum pulses arrive at the bottom nipple (#9, yellow).
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Fuel enters carb
  • #10. Fuel enters carb
  • Fuel enters the carb from the pump by means of a passage through the casting (#10, yellow).
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Upper carb body
  • #11. Upper carb body
  • After removing four screws, the upper body of the carburetor lifts straight up away from the lower body. The float, inlet needle, gasket, and main emulsion tube go with the upper body (#11). Fuel that traveled from the pump, through a passage in the lower body, arrives at the upper carb body at (#11, red).
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Float hinge rod
  • #12. Float hinge rod
  • With the carb upper body upside down the float should sit parallel to the body. If it does not, the float may be stuck or deformed, the rubber tip of the inlet needle may be damaged, or there may be debris between the needle and seat. To remove the float, push the float hinge rod out from one side and pull it out with needle-nose pliers (#12). The float can now be lifted up with the inlet needle attached.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Float and inlet needle
  • #13. Float and inlet needle
  • When the float is lifted out, the inlet needle will come out with it. It is assembled as shown (#13). Inspect the needle’s rubber tip for wear, corrosion or damage. Shake the float to see if fuel has leaked into it.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Inlet valve seat
  • #14. Inlet valve seat
  • Fuel from the lower body arrives at the upper body at #14, red and continues to the inlet needle seat (#14, blue) through a passage in the casting. Fuel is allowed to pass the inlet valve into the fuel bowl until the fuel level rises sufficiently to cause the float to rise, pushing the inlet needle against the seat.
  • PART 2:

Briggs and Stratton Opposed Twin-Cylinder Carburetor - Main jet at bottom of fuel bowl

  • #1. Main jet at bottom of fuel bowl
  • The main jet is screwed into place near the bottom of the fuel bowl (#1, blue). The float and needle valve maintain a constant level of fuel in the bowl. Fuel travels from the bowl through the main jet and up the passage at #1, red, flowing around the emulsion tube where it mixes with air from the main air bleed in the upper body. It then follows a short passage in the upper body and goes down the hole at #1, yellow, and is discharged into the venturi at the main nozzle (#1, white).Fuel from the main jet also supplies the idle, or low speed, circuit, flowing across and up into the idle jet (#1, green). Air from the idle air bleed (#1, purple) mixes with this fuel through the holes in the side of the idle jet/idle emulsion tube. Exiting the jet, fuel travels across a short connector passage in the upper body entering the lower body (#1, pink) on its way to the idle discharge ports.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Main jet accessed from bowl plug
  • #2. Main jet accessed from bowl plug
  • The main jet can be accessed through the bowl plug on the side of the carb (#2, red) without disassembling the rest of the carb.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Main jet
  • #3. Main jet
  • Fuel for both the high and low speed circuits passes through the main jet (#3). Older carbs used an allen wrench, newer ones are slotted.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Fuel bowl vents
  • #4. Fuel bowl vents
  • Two large holes in the top of the upper body (#4, blue) vent the fuel bowl and must not be blocked by the air cleaner or its gasket. The main air bleed (#4, red) supplies air to the inside of the emulsion tube. The air then mixes with the fuel passing by the outside of the emulsion tube.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Main fuel emulsion tube
  • #5. Main fuel emulsion tube
  • The main emulsion tube is part of the upper body (#5) and fits into a hole in the lower body. Air from the main air bleed flows inside the tube to mix with fuel passing around the tube.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Upper body fuel paths
  • #6. Upper body fuel paths
  • Fuel mixed with air, having flowed past the emulsion tube, crosses a short passage in the upper body (#6, yellow) and returns to the lower body on its way to the main nozzle. Fuel from the idle jet crosses a short passage in the upper body (#6, red) and returns to the lower body on its way to the idle disharge ports. Fuel from the fuel pump passes from the lower body into the upper body at #6, blue and continues to the float needle seat (#6, green) and into the fuel bowl.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Low speed jet and emulsion tube
  • #7. Low speed jet and emulsion tube
  • Fuel for the low speed circuit flows into the small hole in the tapered end of the idle jet, shown with a wire, and through the emulsion tube portion of the part (#7). Air from the idle air bleed enters the tube through the holes in its side.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Removing welch plug
  • #8. Removing welch plug
  • Fuel mixed with air exits the idle jet, crosses a passage in the upper body and returns to the lower body. It flows down to a chamber, or mixing well, behind a welch plug (#8). If necessary, a sharpened punch can be used to pierce the aluminum plug and pry it out. Fuel also continues a bit further to a passage just below the mixing well, sealed with a brass ball plug.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Behind the welch plug
  • #9. Behind the welch plug
  • Behind the welch plug there is a chamber where the idle fuel and air continue to mix (#9). The secondary idle fuel discharge port is at the bottom of the mixing well. The primary idle discharge port can be seen after the ball plug has been drilled out – for demonstration only.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Idle fuel discharge ports
  • #10. Idle fuel discharge ports
  • The idle fuel discharge ports deliver fuel into the carb throat. The primary port (#10, blue) operates when the throttle is very nearly closed. The secondary port (#10, green) delivers progressively more fuel as the throttle plate opens.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Throttle plate is chamfered
  • #11. Throttle plate is chamfered
  • The throttle plate is chamfered for a better seal (#11). Mark the plate prior to Disassembly, Cleaning and Repair .
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Idle speed adjustment screw
  • #12. Idle speed adjustment screw
  • Only the idle speed is adjustable on this carb (#12). The throttle stop screw, or idle adjustment screw, limits how far toward closed to throttle plate may travel.
  • Briggs and Stratton Opposed Twin-Cylinder Carburetor - Choke plate removed
  • #13. Choke plate removed
  • With the choke plate removed, the choke stop (#13, blue), main nozzle (#13, pink), main air bleed (#13, red), and idle air bleed (#13, green) can be seen.

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