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

This is a side draft float type carburetor (#1) with a butterfly throttle. It has a fixed main fuel jet but the idle mixture is adjustable. The preliminary setting for the idle fuel mixture screw is 1-1/2 turns out from lightly seated.

The fuel bowl is held in place by the bowl nut. The main fuel jet is screwed into the bowl nut (#2, center). All fuel passes through the bowl nut and main jet (#2, blue).

The fuel inlet fitting consists of a banjo, two gasket washers and a bolt (#3).

With the carb inverted the float should rest parallel to the carb body. If not, the float can be adjusted by bending the tab on the brass hinge (#4, blue inset). The float needle (#4, yellow inset) has a spring loaded pin at its top to more closely regulate the fuel level in the bowl. Note the ring worn into the conical portion of the needle where it mates against the seat. This needle should be replaced.

The float needle seat (#5) unscrews from the carb body. The area where the needle seats must be clean and free from pits or corrosion (#5, red). The emulsion tube or main nozzle (#5, black) unscrews from the carb center pedestal. It has a number of holes along its length which must be clear. Fuel traveling up through the center of the nozzle mixes with air from the main air bleed entering through these holes.

The low speed or idle jet meters fuel arriving from the idle fuel pickup port (#10, red). Fuel passes through the small hole in the end of the jet indicated by a fine wire in the photo (#6, blue) and exits the jet via the side holes (#6, white). The fuel then meets air from the air bleed. The chamfered end of the jet must seat tightly against the bottom of the threaded hole or excessive fuel will leak by. The idle fuel mixture screw (#6, red) adjusts the amount of fuel/air mix delivered to the engine air intake .

The throttle shaft and its bushing shown at left (#7) are badly worn. This condition will likely cause surging and a lean condition as unfiltered air is drawn into the intake. The choke shaft on the right (#7) incorporates a spring and detent ball.

The throttle plate (#8 ) is chamfered for a close fit in the carb throat and must be installed accordingly.

Air enters the idle air bleed (#9, red) and the main air bleed (#9, blue) ports just before the venturi.

Fuel for the low speed circuit enters the idle fuel pickup hole (#10, red) just before the main fuel emulsion tube. It travels up a passage along the side of the center pedestal sealed by a ball plug (#10, blue)

Fuel that enetred the idle fuel pickup (#10, red) travels up a passage (#11, blue line) along the side of the center pedestal and joins another passage drilled from above (#12, blue).

Fuel from the idle fuel pickup (#10, red) passes through the small hole in the end of the pilot jet (#6, blue) mixes with air from the idle air bleed (#9, red) and continues to the idle progression holes (#13, blue) and the idle fuel port (#13, red) via #12, green lines.

The idle fuel discharge port (#13, red) is directly across from the mixture screw location (#12, pink).

The idle fuel progression or intermediate ports (#13, blue) are directly across from the mixing well plug (#12, white).

A mix of fuel and air for low speed operation is metered by the idle fuel mixture screw and delevered through the idle fuel port (#13, red) when the throttle is nearly closed. As the throttle opens an increasing amount of fuel/air is drawn through the progression or intermediate holes (#13, blue) and enters the intake air stream.

Air from the main air bleed port (#9, blue) just before the venturi joins a passage drilled and plugged at (#14, blue) which continues to the pedestal via the path shown in #15, yellow line.

The yellow line (#15, yellow line) indicates the path between the main air bleed port (#9, blue) and the discharge point (#16, red) outside the emulsion tube.

The main bleed air enters the area outside the emulsion tube at (#16, red) and enters the tube via the bleed holes (#5, black) along its length and mixes with the main fuel supply. The fuel/air mixture then exits the main nozzle (#17, black) into the intake air flow at the venturi.

The fuel/air mix exits the top end of the emulsion tube (#17, black), also called the main nozzle, and enters the intake air stream.