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

Carburetors used on Lawn-Boy lawnmowers (vertical crankshaft) have an air vane (#1, green) connected directly to the carb throttle. As engine speed increases, the flywheel fan blows air on the vane, which moves the throttle to a slower position and vice versa, thus governing the engine speed. A thumbwheel (#1, yellow) is provided for adjusting a torsion spring to achieve the correct engine speed. The choke is operated by a lever (#1, pink) that is controlled by a cable.

Carbs used on two-cycle snowthrowers can have the air vane mounted remotely and connected to the throttle by a link rod. The choke is operated manually. Fuel enters the carb at the inlet nipple (#2, yellow). A remote primer is connected to the primer nipple (#2, white) by a rubber tube.

With the bowl removed and carb held inverted the float should rest parallel to the carb body (#3). The float can be adjusted by bending the brass hinge.

A fine mesh screen keeps out small particles (#4, green). Three prongs attach the float to the hinge (#4, blue). Check the hinge and prongs for cracks. The inlet needle or float needle is attached to the float by a wire clip (#4, pink). Slide out the hinge pin (#4, yellow) to remove the float. The needle will come out with it.

The inlet needle is attached to the float as shown (#5). Check that the rubber tip of the needle is not worn, off-center, or otherwise damaged.

Shown are main nozzle/emulsion tube (#6, green), main jet (#6, red), bowl vent (#6, yellow), primer passage (#6, pink), inlet seat (#6, blue), and fuel inlet tube (#6, white).

Shown are main nozzle/emulsion tube (#7), main jet (#7, blue), inlet seat (#7, yellow), and idle (pilot) jet (a fine wire shows the tiny hole in the end of the jet)(#7, red).

The choke plate is installed with the single tab in first and all tabs outward (#8).

Use a small screwdriver to spread the choke shaft away from the raised tab on the choke plate and pull out the choke plate (#9).

The choke spring (#10) (used on lawnmowers) holds the choke open.

With the choke removed: choke stop (#11, blue), idle circuit air bleed (#11, white), main nozzle (top of emulsion tube) (#11, red), main air bleed (#11, green), bowl vent (#11, yellow). This vent passage is typically sealed with a ball plug on snowthrower applications. It connects to #6, yellow.

The emulsion tube screws into the center tower and the main jet screws into the bottom of the emulsion tube. All fuel passes through the main jet. At high speed fuel is drawn up through the emulsion tube, mixes with air from the main air bleed (#11, green). At idle speed fuel enters the idle fuel pickup hole (#12) within the tower, shown with a wire.

The opposite end of the idle fuel pickup hole is plugged at #13, yellow. The bottom of this side tower is plugged at #13, red. The idle fuel goes up the side tower to the pilot jet.

The idle, or pilot, jet is screwed into side of the carb (#14, yellow). Idle fuel enters the jet through the tiny hole (#7, red) in the chamfered end. The chamfer must seat tightly into its seat in the carb body or excess fuel will be drawn in. Fuel exits the jet through the larger side holes and air from the idle air bleed (#11, white) mixes with it.

Fuel and air from the idle jet arrive at the idle fuel mixing well (#15, yellow). A plug covers the well.

The idle fuel discharge ports (#16, white) are inside the carb bore, directly across from the idle fuel mixing well. The primary idle fuel port is exposed to intake vacuum when the throttle is nearly closed, and as the throttle opens the secondary port also supplies fuel.

The throttle plate is installed with the raised dimples facing inward (#16, blue) and the relief hole near the idle fuel ports.

A ball plug (#16, green) seals the back end of the idle fuel/air passage.

Note that the mounting flange does not appear to be flat. These carburetors are notorious for deforming and not sealing well against the engine block. Over-tightening the carb to get a good seal causes the throttle to bind.

Lawnmower (vertical crankshaft) carbs provide a thumbwheel by which the governor spring can be adjusted (#17).

Snowthrower (horizontal crankshaft) carbs typically have the air vane mounted remotely and engine speed is adjusted by bending a tab to which the governor spring attaches (#18).