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

The Briggs and Stratton Tank-mounted Pulsa-jet Fixed-jet carburetor shown here can be identified, without removing the carb from the tank, by the presence of the fuel pump cover on the side of the carb and the welch plug above the needle valve. This carb is used on engines with horizontal crankshafts.

The presence of threaded holes on the intake flange indicates a carb with a fixed main jet (#2).

With the carb removed from the tank, Pulsa-jet carbs can be seen to have two fuel pickup tubes. The shorter pickup tube is part of the carb casting in the fixed-jet version. In the adjustable-jet version, the shorter pickup tube is plastic and screws into place. For this particular carb, the main jet is located on the side of the shorter fuel pickup tube and is fixed (not adjustable) (#3).

The newer version of the Pulsa-jet carburetor has a fixed jet at the bottom of the pedestal (#4).

The term 'Pulsa-jet' refers to carbs that have a fuel pump operated by pulses in intake pressure. #5 shows the pump cover on the side of the carburetor.

The longer fuel pickup tube extends down to the bottom of the fuel tank. A screen at the end of the plastic extension keeps dirt out (#6). Fuel is drawn up this tube to the fuel pump on the side of the carburetor.

The brass pipe portion of the pickup is a press fit into the carb casting and should not be removed (#7). The plastic section can be removed by sliding the metal spring clip onto the brass section and cutting the plastic tube where it meets the brass tube. To install a new plastic tube, first slide the new retainer clip onto the brass tube. Heat the new plastic tube in hot water and press it onto the brass tube. Then, slide the retainer clip into position on the plastic tube.

The copper wire inserted into the hole where the longer fuel pickup tube was removed shows the passage to the upper hole in the pump (#8).

Flappers on the pump diaphragm act as check valves preventing backflow of fuel (#9).

Fuel flows past the first flapper into a recess in the pump cover and out past the second flapper valve (#10).

Fuel passes out past the second flapper valve (#11) and into a well on the fuel tank (shown with long tube removed). Excess fuel in the well overflows into the main part of the fuel tank thus maintaining a constant level of available fuel .

Fuel leaving the pump fills the reservoir well in the fuel tank (#12). Excess fuel in the well overflows into the main part of the fuel tank thus maintaining a constant level of available fuel.

Pulses in engine intake vacuum draw the pump diaphragm inward. A spring returns the diaphragm outward. A cup covers the spring to prevent damage to the thin rubber diaphragm (#13). The in and out motion of the diaphragm working in concert with the flapper check valves causes fuel to flow through the pump.

The shorter of the two pickup tubes is cast as part of the carb body. This tube picks up fuel from the reservoir in the tank and delivers it to the high and low idle circuits. In #14 the welch plug at the end of the tube has been removed. Highlighted in green is the fixed side jet. The lower end of the brass emulsion tube is shown by yellow. The opposite end of this tube is the main nozzle. Red indicates the low idle fuel pickup hole. Blue shows a ball plug which seals a passage that connects the low idle pickup hole to the idle mixture needle valve.

Fuel from the idle pickup hole arrives next at the idle mixture adjustment needle valve. The initial setting for the idle mixture is 1-1/2 turns out from lightly seated (#15). Check the needle tip for a straight smooth taper.

A ball plug seals a passage drilled from the needle valve to the idle fuel mixing well (#16).

The welch plug has been removed from this idle fuel mixing well (#17). The hole shown in blue brings fuel from the needle valve. The red hole is the idle air bleed and brings air from inside the carb throat to mix with the fuel. The three holes in green are located just before and after the throttle closed position and successively deliver the fuel/air mixture into the carb throat as the throttle opens.

A large welch plug closes the carburetor at the elbow (#18). It can be popped out from the inside with a screwdriver and a sharp blow. When installing a new plug, set it in place, flatten it somewhat with a hammer and stake it, or peen over the carb body, around its circumference in several places. Seal it with nail polish.

Looking into the carb throat with the welch plug removed (#19), the red square indicates the top end of the emulsion tube, also known as the main nozzle. As fuel is drawn up through the emulsion tube, air is pulled through the main air bleed indicated by the green square to an area surrounding the emulsion tube. Air enters the tube through a number of holes along its length, breaking the fuel up into small droplets which then enter the venturi at the nozzle (red). The hole indicated by blue is the idle air bleed and delivers air to the idle mixing well.

The idle speed screw determines how far toward the closed position the throttle is allowed to go (#20).

The throttle shaft seal is installed as shown with the lip inward if the throttle plate is stamped from sheet metal, upward if the plate is cast metal (#21).

Gases that exit the crankcase breather are recirculated into the carb through the breather opening (#22) to reduce pollution.

This throttle link lever is installed as shown (#23).