The Briggs and Stratton Vacu-jet Automatic choke carburetor shown here can be identified by the small 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 model carb has a small breather opening (#3) which measures about 1/2 inch. The 'All Temperature' carb has a breather opening that measures about 1 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. 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 positioned just before the throttle plate. The smaller one supplies fuel for low idle and is positioned 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 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 red (#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.
The area shown in red (#12) must seal against fuel leaking to the outside or into the vacuum chamber shown in the previuos photo (#11).
The diaphragm link rod attaches to the choke shaft/lever (#13). The choke link cover prevents the link from coming out of place. When assembling the carb to the tank the 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, tighten the screws evenly and securely. The choke plate should move freely and close fully due to the diaphragm spring.
The choke shaft/lever and the choke plate snap together (#14). Use a screwdriver to press the shaft off the plate.
After removing the choke shaft, to remove the choke plate move it to the wide open position and pivot it and lift it out as shown (#15).
The raised tab of the throttle plate is aligned toward the fuel metering holes (#16) and the raised part facing toward the engine.
An o-ring seals the union to the engine intake pipe (#17). 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.
The o-ring retainer snaps into position and can be removed if necessary (#18).
The idle speed screw determines how far toward the closed position the throttle plate can travel thereby setting the low idle speed (#19). Idle is normally 1750 rpm.