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

Unhook the extension spring (#1, green) from the governor lever. Unclip the bushing (#1, pink) at the bottom of the throttle link rod and slide the rod out of the governor lever.

Carbs that have a fuel shutoff solenoid will have a ground wire (#2, green). The bushing at the upper end of the throttle link fits only one way without binding (#2, blue). The tops of both throttle and choke links hook inward from the outside (#2, pink).

Slide the carb away from the engine and unhook the bottom of the choke rod. The tops of both links can now be removed from the carb.

Fuel enters the carb at the fuel inlet pipe (#3, blue). Strong vacuum when the throttle is nearly closed is delivered to the accelerator pump (#3, green) via the vacuum hose (#3, red). Fuel exits the pump and flows to the carb intake via the accelerator fuel delivery tube (#3, yellow).

Fuel from the accelerator pump is discharged into the carb intake at the accelerator fuel nozzle (#4, black). The main fuel nozzle (#4, pink) extends up into the venturi. The idle fuel transfer tube (#4, white) goes from the fuel bowl, through the emulsion tube and main nozzle, to the top of the carb. The main air bleed jet is at #4, blue and the idle air bleed is at #4, yellow. The bowl vent is at #4, green.

The fuel shutoff or anti-afterfire solenoid (#5) is shown. The inset shows a close-up of the main jet, which, in this case, screws into the top of the solenoid assembly.

Viewing the inside of the fuel bowl, a wire shows the location of a bleed jet (#6, red). Fuel enters the accelerator pump through a large hole (#6, blue) near the bottom.

The accelerator pump body is part of the fuel bowl. Fuel from the fuel bowl enters the pump via a hole at #7, pale blue. A check valve (#7, blue) sets into a recess (#7, yellow). A bleed hole is at #7, pink. Fuel exits the chamber at #7, black. A check valve pressed into the body (#7, white) completes the system.

A compression spring pushes against the cupped side of the metal center plate of the diaphragm and the flat side goes against the adjusting screw (#8 inset).

When the engine is running at a low speed, the throttle plate is nearly closed and strong vacuum is created behind it. A hole (#15, white) at this location supplies vacuum to the spring side of the pump diaphragm, overcoming the spring, causing fuel to be drawn across the rubber inlet check valve into the body. As the throttle is opened, the vacuum reduces and the spring pushes against the diaphragm forcing fuel out past the second check valve to the carb intake.

Carbs that do not have a fuel shutoff solenoid will have the main jet screwed into the center pedestal (tower) at the location shown by #9, red. The idle fuel pickup and transfer tube (#9, yellow) is within the main emulsion tube (#9, blue). The idle fuel tube must be removed prior to the emulsion tube.

The inlet needle is assembled to the float (#10). Inspect the needle for wear and corrosion and that the rubber tip is centered and true.

The rubber tip of the inlet needle must make a good seal with the inlet seat (#11, blue) or the engine will run rich, flood or the crankcase may fill with fuel over time.

The main nozzle/emulsion tube (#12, left) has a number of holes (#12, red) along its length that allow air from the main air bleed to enter and mix with the fuel that is drawn upward inside the tube. The top end of the emulsion tube is the main nozzle (#12, yellow) where fuel is delivered into the incoming air stream at the venturi. The bottom of the idle fuel transfer tube is the idle fuel pickup (#12, blue). The slow or idle jet (#12, white) is at the top. Check the idle fuel mixture needle (#12, green) for damage or a groove in the tapered tip.

Idle fuel comes up through the slow jet (#13, pink). Air from the idle air bleed comes up at #13, white and mixes with the fuel. When the throttle is nearly closed, fuel is routed to the primary idle fuel discharge port via the passage at (#13, red). As the throttle opens slightly fuel is drawn from the idle progression holes (#13, light blue).

Carbs that are equipped with an idle mixture screw may have a small orifice leading to the mixture valve (#11, red).

The preset for the idle fuel mixture adjusting screw (#15, red) is 2-1/4 turns out from lightly seated. A ball plug (#15, blue) seals a passage used during manufacture. Locations for passage variations are cast into the body (#15, yellow).

Primary idle fuel discharge port (#16, blue); vacuum port for accelerator fuel pump (#16, white), secondary or progression fuel discharge ports (#16, red).

The idle speed screw (#17, blue) should be set at 2-1/4 turns from closed. The optional throttle stop (#17, red) limits the amount that the throttle may open. 'Nikki' logo (#17, black).

This carb features a self-relieving choke (#18). When the engine starts the high vacuum behind the closed choke overcomes the choke spring, thus preventing excess choking.