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

The fuel inlet pipe (#1, pink), idle pilot jet (#1, yellow) and the idle mixture screw (#1, blue) are highlighted in the photo.

The brand 'Mikuni' is cast into the body of the carburetor (#2, yellow).

The bowl drain screw can be removed to drain the carb bowl for storage (#3, yellow). Remove the bowl nut (#3, red) to take off the bowl. Be prepared to catch the fuel that leaks out in a suitable container.

After removing the fuel bowl, the float, center pedestal and main fuel jet are visible (#4). All fuel passes through the main jet into the center pedestal. The float maintains a constant fuel level in the bowl.

The float hangs from the carb body by the hinge rod (#5). To remove the rod, grasp the flat head with a twisting and pulling motion.

After removing the float hinge rod, the float and inlet needle can be lifted out (#6). The needle attaches to the float with a clip as shown. As the fuel level rises in the bowl, the float pushes the rubber-tipped needle against the inlet seat, thus stopping fuel flow. Check the needle tip for damage or wear. A spring loaded plunger on top of the needle (#6, blue) allows for finer control of the fuel level.

The inlet needle, or float needle, is pushed by the float into the seat (#7) as the fuel level rises. Check the seat for corrosion or pitting and for dirt.

Fuel that has passed through the main jet goes either into the main nozzle / emulsion tube (#8) or into the idle fuel pickup, both of which are within the center pedestal. Air from the main air bleed jet enters through the small holes in the side of the emulsion tube portion of the part, and mixes with the fuel. The fuel / air mixture rises through the tube to the nozzle end where it enters the venturi airstream.

At the intake end of the carb there are two removable air bleed jets: the main air bleed jet (#9, yellow) and the idle, or pilot, air bleed jet (#9, blue).

A wire shows the location of the idle fuel pickup (#10, blue) within the center pedestal. The inset photo shows a view of the idle fuel pickup looking through the main jet hole (#10, yellow). Fuel that enters the idle fuel pickup enters a channel, the lower end of which is sealed with a ball plug (#10, green).

Fuel from the idle fuel pickup enters the pilot jet through the small hole in its end (#11). The tapered end of the jet must seat tightly into the carb body so that fuel enters ony by the small jet orifice. Air from the idle air bleed jet enters through the larger holes in the side of the pilot jet and mixes with the fuel. The mixture continues to the idle mixture screw and the idle fuel discharge ports.

A fuel/air mixture from the pilot (idle) jet arrives at the idle fuel mixture needle (#12). The needle meters how much of the mixture can enter the carburetor airstream. The initial setting for the needle is 1-3/4 turns out from lightly seated for EX13, 1-1/4 turns for EX17, 1 turn for EX 21. An inset photo shows the plastic limiter cap removed from the needle. Another inset (#12, blue) shows a view looking into the needle screw hole at the primary idle discharge port.

The primary idle fuel discharge port (#13, yellow) is metered by the idle mixture screw. The progression holes (#13, blue) deliver increasingly more fuel/air as the throttle opens.

The throttle plate has chamfered edges for a smooth fit in the carb bore (#14, yellow). Mark it prior to Disassembly, Cleaning and Repair .

The throttle and choke shafts have serviceable bushings and seals (#15).

The idle speed screw limits how far toward closed the throttle may travel, thereby setting the idle speed (#16).