Outdoor Power Equipment, Small Engine, Lawn Mower Repairs, Troubleshooting and Safety

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.