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

The Briggs and Stratton Vacu-jet Automatic choke All-temperature carburetor shown here can be identified by the large breather opening (#1, red ) and the choke link cover (#1, green ). It has only one fuel pickup tube extending from the bottom of the carb to the fuel tank (#8 ), whereas the pulsa-jet carb has two.

The breather opening on the carb is connected to the crankcase breather by a tube and two rubber elbows (#2). Gases that blow by the piston rings or valves are directed to the carb intake to reduce pollutant emissions. If either elbow is cracked or missing, unfiltered (dirty) air will be drawn into to carb.

This 'All Temperature' model carb has a large breather opening (#3) which measures about 1 inch. The standard carb has a breather opening that measures about 1/2 inch.

A single needle valve meters the fuel flow for both high and low idle (#4). The initial setting is 1-1/2 turns out from lightly seated. From initial setting, turn screw in until engine misses, then back out screw 3/8 turn.

This carb has a 'pressed-in' type of needle valve assembly (#5). It is held in place by the flat rubber washer which is squeezed outward by the pressure of the spring acting on it. To remove the assembly, back out the screw to relieve pressure on the rubber washer, leaving about 3-4 turns of the screw in the plastic seat, and pull the assembly out. Alternatively, remove the screw, spring, metal washer and rubber washer. Re-install the screw and pull out the seat. The o-ring might remain in the recess and need to be fished out.

An exploded view of the needle valve assembly (#6).

There are two metering holes at the end of the needle valve bore (#7). The larger one is the main fuel supply and is located just before the throttle plate. The smaller one supplies fuel for low idle and is located just beyond the closed throttle position. The fuel is pulled up from the fuel tank through the fuel pickup tube, through a hole in the bottom of the needle valve bore, past the needle and seat, and then to the metering holes.

Looking at the bottom of the carb you can see the fuel pickup tube through which fuel passes to the needle valve assembly (#8). The diaphragm and spring are also seen.

The fuel pickup tube snaps into the bottom of the carb with considerable force, but can be removed and replaced manually (#9). There is a ball check valve in the bottom of the tube to prevent fuel draining back down. Verify that the ball is free by shaking the tube up and down. Ensure that fuel can pass upward but not downward. Be sure the screen is clean.

The choke spring is attached to the diaphragm by a clip as shown (#10). Check this area of the diaphragm carefully for wear or holes.

The area shown in green (#11) must be an airtight seal so that vacuum from the intake pipe can be applied to the diaphragm. Check this area of the tank for flatness. Any fuel found in the diaphragm spring well is a sign of leakage of both fuel and vacuum.

Vacuum from the engine intake passes along a channel in the fuel tank and through a small restrictor orifice (#12). Check that this hole is clear.

The area shown in red (#13) must seal against fuel leaking to the outside or into the vacuum chamber shown in #11. The fuel pickup tube snaps into the hole in the center of the highlighted area, passes through an opening in the diaphragm, and reaches to the bottom of the fuel tank.

The diaphragm link rod attaches to the choke shaft/lever. #14 shows the link and lever in the closed choke position. The choke link cover prevents the link from coming out of place. When assembling the carb to the tank, the auto-choke diaphragm must be preloaded to ensure there is sufficient slack in the diaphragm to allow a full range of movement. To preload the diaphragm place the carb in position on the tank and install the screws loosely so the carb is free to move. Next, move the choke lever to a position past wide open as far as it goes, insert link rod into choke lever hole. Now, while holding the choke in this open position, tighten the screws evenly and securely. The choke plate should move freely and close fully due to the diaphragm spring.

The bimetal spring (#15) responds to the heat of the crankcase breather gases being drawn into the carb. As the engine warms up the bimetal tends to unwind. When the engine is cold the choke is held closed providing a richer fuel mixture. As the engine heats up less choke is applied. This feature is why the carb is termed 'all-temperature'.

The choke plate and the two choke shaft pieces snap together (#16). Use a screwdriver to press either shaft off the plate. Some carbs may have a short anchor post for the bimetal spring end loop that has been flared to prevent the spring from coming off. If so, file it down to remove the spring.

The bimetal/shaft assembly, the choke plate and the choke lever/shaft are shown in #17. To reassemble, hold the choke plate in a vertical position in the carb throat with needle-nose pliers. Align the lever/shaft so that the lever link hole is near the 7 o'clock position and slide the notched end onto the plate until they snap together. With the choke plate in the closed position, align the bimetal/shaft so that the end loop is in the left half (toward the engine) of the breather opening, assuming the ambient temperature is between 40-120 °F. Slide the bimetal/shaft notch part way onto the choke plate and while holding it in place, put the spring loop on the anchor post. Now, snap the pieces together. Some carbs with a short anchor will require that the top of the post be flared with a warm soldering iron to keep the bimetal spring in place.

The raised tab of the throttle plate is aligned toward the fuel metering holes (#18) and the raised part faces toward the engine.

An o-ring seals the union of the carb to the engine intake pipe (#19). When assembling the carb to the engine put some oil on the intake pipe and/or the o-ring to allow the parts to slide together without distorting the o-ring. Check the intake pipe for looseness or cracks.