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

The LMT carb can be identified the letters 'LMT' cast into the carb body (#1).

This Walbro carb is equipped with an anti-afterfire solenoid (#2, yellow). It also has an idle adjustment limiter cap (#2, green). The idle jet is pressed in and the access hole sealed with a ball plug.

This earlier LMT carb has a simple bowl nut instead of a solenoid (#3, yellow), a fully adjustable idle mixture screw (#3, green). It also has a screw-in type of idle jet (#3, red). Some LMT carbs may have adjustable main jets rather than a bowl nut or solenoid.

When the engine is shut down the spark is disabled and the engine coasts to a stop. While the engine is coasting, fuel is still being drawn in from the carb, and the unburned fuel continues out to the hot muffler where it may explode loudly. The anti-afterfire solenoid (#4) stops fuel flow into the engine as soon as the keyswitch is turned off. The plunger or pintle is extended at rest and retracts when supplied with 12 volts.

To remove the fuel bowl unscrew and remove the solenoid, bowl nut or main needle assembly as needed. The float hinge rod slides out either side (#5) and the float with inlet needle can be lifted out.

#6 gives a view of how the float needle is assembled to the float. It just slips into place. The inset shows needles with crooked rubber tips and one with corroded plating.

The inlet seat is replaceable and can be pulled out with a screw extractor (#7). A better method is to improvise a puller. Assemble a 1/4 - 20 screw, a flat washer, a nut, and a 1/4 in. drive 3/8 socket as shown in the photo. Tap a 1/4 - 20 thread into the seat, screw the puller assembly into the seat, then as you tighten down the nut, the seat will be pulled out. The new seat should be pressed in until it bottoms or the top sits flush to the body, whichever comes first.

#8 shows the float area after the float has been removed. The main nozzle can be seen in the center post.

Carbs fitted with an anti-afterfire solenoid will have the main jet screwed into the side of the center pedestal (#9). It must be removed prior to removal of the main nozzle.

Carbs not fitted with an anti-afterfire solenoid will have the main as part of the nozzle / emulsion tube (#10).

Remove the main nozzle with a screwdriver that fits the nozzle's slot well. The nozzle is made of brass and it's easy to strip out the slot (#11).

The main nozzle has a number of holes that must be clear (#12) for good operation. This area may be called the emulsion tube. During operation air from the main air bleed is drawn through these holes atomizing the fuel into small droplets.

The brass insert shown in #13, green is the main air bleed. It supplies air to the emulsion tube to atomize fuel being drawn into the venturi from the nozzle. #13, yellow shows the idle circuit air bleed jet. It connects directly to the idle jet and idle mixture screw. The large passage shown in red connects to the vents on the opposite end of the carb and the bowl vent above the baffle.

View of float area with baffle plate removed (#14). There is probably never a need to remove the baffle. Wires show vent passages to throttle end of carb. Yellow indicates an additional vent that connects to the others.

On the side of the center post is a ball plug highlighted in #15. Opposite this plug, inside the post, is a hole which picks up fuel for the idle circuit.

In this cutaway view, inside the center pedestal, opposite the ball plug is the idle fuel pickup port (#16). From here fuel goes to the idle fuel jet.

Older LMT carbs may be fitted with a fully adjustable idle mixture screw (#17, yellow) and a screw-in type idle jet, or pilot jet (#17, blue and inset #17, pink). Fuel from the idle fuel pickup port enters through the end hole of the pilot jet and mixes with air from the idle air bleed. The jet screws in tight and must seat against the bottom of the threaded hole. The initial setting for the idle fuel mixture screw is 1-1/4 turns out from lightly seated. The idle speed screw is shown at #17, green .

Newer LMT carbs have a limiter cap on the idle mixture adjustment screw (#18, yellow). A ball plug and pressed-in idle jet replace the screw-in type of idle jet (#18, blue). A ball plug (#18, red) seals the passage drilled to the idle fuel mixing well.

After drilling out the ball plug the pressed-in idle jet can be seen (#19). This was only drilled out for demonstration and is not normally done as part of service.

To remove the idle mixture screw the limiter cap must be removed. It can be pulled straight off with pliers (#20).

The limiter cap fits tightly over a knurled portion of the screw (#21).

Low speed screw removed, needle tip visible (#22).

To remove the welch plug from the idle mixing well pierce it with a sharp tool and pop it out. These holes lead to the carb bore just prior to the closed throttle plate position (#23).

Low speed air and fuel orifices (highlighted) near throttle closed position are directly connected (#24).