The Walbro model number WT340 is stamped on the side of the carburetor body.
Shown here is a view of a typical engine to which the Walbro WT-340 might be mounted. As the piston moves up and down in the crankcase, the pressure in the crankcase rises and falls with each stroke. These pulses of pressure are connected to the carburetor via a passage (#2, A) from the crankcase through the carburetor gasket.
The pressure pulses from the crankcase are used by the carb to power a fuel pump. A hole (#3, A) on the side of the carb aligns with the port (#3, B) on the pump side of the carb. From there the pulse goes through the gasket and a passage in the pump cover at (#3, C) to a chamber above the diaphragm (#3, D). At this point the crankcase pressure pulses cause the diaphragm to move up and down. Fuel located in a reservoir (#3, E) on the opposite side of the diaphragm is acted upon by the motion of the diaphragm.
Fuel from the fuel tank enters the carb at the fuel inlet pipe (#4,A) which connects to the hole marked 'B' (#4, B). Here the fuel flows past the flapper valve (#4,C), passes along a cutout (#4, D) in the gasket where it enters a hole and passage (#4, E) arriving at the fuel reservoir (#3, E).
While in this reservoir the fuel is pushed and pulled by the action of the fuel pump diaphragm which is being driven by the pressure pulses from the crankcase. When the pressure increases in the reservoir, fuel is prevented from flowing backwards by the flapper (#4, C) closing against hole 'B' (#4, B) so it must flow out through the passage at (#4, F) and past the flapper (#4, G). The fuel then travels via a cutout (#4, H) in the gasket, enters a hole (#4, I) that connects (#4, J) to the area above the inlet screen (#4, K).
The inlet screen (#5) is a fine wire mesh which acts a fuel filter. It can be pulled out with a sharp pointed pick for cleaning. The screen is the most frequently clogged part of the carb. Also, once the screen is wetted with fuel mix, water will not pass through it, so a single drop of water will block flow. The screen should sit squarely on the bottom shoulder of the recess with its sidewalls snug to the second shoulder wall. Seen below the screen is the back side of the metering needle seat.
Fuel that has passed through the inlet screen travels to the metering side of the carburetor through the metering needle seat (#6, A), shown here with the needle removed. On the opposite side of the metering chamber there is a passage (#6, B) to the fuel purge pipe (#6, C), through which fuel is drawn out to the primer bulb and further to the fuel tank during purging. Fuel that will run the engine enters the check valve (#6, D)
The metering needle (#7, A) hooks onto the lever (#7, B) which pivots on a pin (#7, C). A spring (#7, D) sitting under the pivot lever pushes the needle to the closed position. A single screw (#7, E) holds the assembly in place. The top end of the spring must be centered over the dimple (#7, F) on the underside of the metering lever, while the lower end sits squarely in the pocket in the carb casting.
Assembly: First set the spring in the pocket. Grasp the needle, lever and pin assembly and lower it onto the spring while guiding the needle to the seat (#8).
Be sure the spring is centered over he dimple on the underside of the lever and squarely set into its pocket. Check that the pin is located in the slot. The screw should tighten on the pin, almost flush to the casting. Ensure that the needle and lever move freely and are under load from the spring (#9). Lay a straightedge across the entire width of the carb body and adjust the metering lever so that its free end is 0.060 - 0.070 inch below the carb body (WA, WT, WTA carbs with welch plugs, not circuit plate type).
The welch plug is a convex aluminum disk (#10) that covers several passageways machined into the casting. Although seldom required, it can be removed by piercing it with a sharp awl or pick and prying it out, being careful not to damage the passages it covers. Sealant is not usually necessary but nail polish is commonly used. Lay the plug in position, apply a small amount of nail polish around the circumference, drive the plug flat with a punch somewhat smaller in diameter than the welch plug, then wipe off the excess polish. The welch plug must be punched flat enough to ensure that it does not interfere with the operation of the metering diaphragm.
Fuel that has entered the metering chamber side of the carb can leave by only two ways. The first is via a passage (#6, B) to the outlet pipe (#6, C) that leads back to the fuel tank. This only occurs when purging the carb with the primer bulb.
All fuel for running the engine passes through a single check valve (#6,D) in the metering chamber. That check valve is shown here in more detail (#11). A flat rubber disk in the valve prevents air from the carb throat from entering the chamber while fuel is drawn out of the chamber during purging. The check valve can removed by pulling it out with a screw extractor, but not without destroying it.
The check valve which would be located at #12, X has been removed in this photo. Fuel for both the idle and main circuits passes through the check valve, enters a passage (#12, 1) that leads to the main fuel adjusting needle (#12, 2) , and then to a small reservoir (#12, 3) above the main nozzle. From this reservoir, fuel may take either of two paths. The first - the main fuel path - is through the main nozzle, into the venturi and on to the engine. The second path - the idle fuel circuit - leads to the idle fuel adjusting screw (#12, D) , and then to the idle fuel discharge ports (#12, F). Because the idle fuel passes through the main fuel adjustment, it is called a dependent circuit.
The initial setting for both fuel adjustment needle screws is 1-1/4 turns out from lightly seated (somewhat rich), but may vary depending on the application.
A ball plug (#13, A) shows where a drill entered the carb, continued above the main nozzle to #13, B until it met the idle fuel screw hole at #13, C. From here the fuel is metered by the needle screw (#13, D) and enters the idle fuel well at #13, E. From the well, fuel exits into the engine intake airstream via the idle fuel discharge ports (#13, F).
The idle fuel disharge ports (#14, A) are located in the carb throat near the throttle plate. The primary idle port is closer to the engine than the nearly closed throttle and supplies fuel at the lowest engine speeds. As the throttle opens, fuel is increasingly drawn from the farther secondary ports.
The throttle plate is oriented as shown (#15).
The main nozzle contains a flat disc that acts as a check valve, preventing air from the carb throat from entering the idle fuel circuit during low speed operation.
Fuel exits the main nozzle (#17, A) into the venturi for high speed engine operation. The nozzle can be removed by pressing it out into the carb throat (in some cases the high speed needle may need to be removed first).
Pump side of carburetor: Diaphragm (#18, A) is next to carb body, followed by the gasket (#18, B) and cover (#18, C).
On the metering side of the carb: Gasket (#18, D) goes next to carb body, followed by diaphragm (#18, E) with the large stiffener plate and button facing the metering lever, and cover (#18, F). A small hole (#18, G) in the metering cover allows atmospheric pressure to act on the diaphragm. When assembling the cover to the carb, the hole should be positioned so as to minimize the possibility of becoming clogged. Generally, the hole should be oriented downward in normal operation. If the engine flywheel fan or other component blows air past the carburetor, the hole should positioned so dirt, chips or debris are not blown into it, and so air movement does not alter the pressure over the diaphragm.