Stets für ausreichend Belüftung sorgen, Benzindämpfe sind krebserregend.
3-bolt venturi is about .870 – .875,
4-bolt is about .780 Diameter.
(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.
- ‚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.
- 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.
- Behind the pump cover plate is a gasket and then a diaphragm (#3). Ensure that the diaphragm is not perforated, deformed, brittle or gummy.
- 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.
- The main pump diaphragm (#5) is between the pump body and the carburetor. Ensure that the diaphragm is not perforated, deformed, brittle or gummy.
- 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).
- There is a small vent at the bottom of the vacuum chamber (#7) that should not be obstructed.
- 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).
- 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).
- Fuel enters the carb from the pump by means of a passage through the casting (#10, yellow).
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- The throttle plate is chamfered for a better seal (#11). Mark the plate prior to Disassembly, Cleaning and Repair .
- 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.
- 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.