The Walbro model number WT631 is stamped on the side of the carburetor body. Fuel from the fuel tank enters the carb at the fuel inlet pipe (#1, A). Fuel circulated through the carburetor during purging exits the outlet pipe (#1, B) mounted in the pump cover and returns to the fuel tank.
Shown here is a view of a typical engine to which the Walbro WT-631 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, 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. Below the screen is the back side of the metering needle seat. When seated correctly, there will be a gap between the screen and the seat below it.
On the metering side of the carb there are the welch plug (#6, A), metering needle assembly (#6, B), check valve (#6, C), and a ball plug (#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. A drop of oil or a dab of grease may help stick the spring in place. 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, B and insets) 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 hole (#10, A) under the metering lever assembly that leads to the purge bulb on the pump cover. This only occurs when purging the carb with the primer bulb. All fuel for running the engine passes through a single check valve (#10, C) in the metering chamber.
Fuel that has entered the metering chamber side of the carb can leave by only two ways. The first is via a passage (#10, A) that leads to the purge bulb on the pump cover. This only occurs when purging the carb with the primer bulb.
All fuel for running the engine passes through a single check valve (#10, C) 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.
Fuel that has passed the check valve (#12, A) must then pass the fuel mixture adjustment screw (#12, superimposed) and its seat. The adjustment screw meters both the hi and lo speed fuel flows. Having passed the adjusting screw fuel may take either of two paths for hi speed or lo speed operation. A brass ball plug (#6, D) seals a drilled passage to the venturi for fuel used at hi speed. This is the main jet. Another drilled passage (#12, D) intersects the main jet passage and leads to the idle fuel well (#12, E). Fuel for lo speed operation is pulled into the carb throat through the discharge port (#12, C) when the throttle plate is near closed position.
Air is drawn in from the air bleed port (#13, A) just before the venturi through a hole (#12, B) drilled from the idle fuel well and mixes with the fuel flowing to either the main (hi) or idle (lo) fuel discharge jets.
A drilled hole, the air bleed port (#13, A), allows air from a point just before the venturi to reach the idle fuel well (#12, B) from which it can mix with the hi and lo fuel flows.
The idle fuel discharge port (#14, A) and the main fuel discharge port (#14, B) deliver fuel into the carb throat from #12, C and #6, D, respectively .
The path for the purge circuit diverts from the main fuel flow in the metering chamber via a hole (#15, A). It will exit the carb via the passage (#15, B) and outlet pipe (#15, C) after going through the purge bulb and check valve.
Purge fuel comes up through a hole (#16, A) lifting the skirt of the check valve and arrives in the bulb. The flexible rubber check valve presses snugly into the center hole (#16, B) of the cover. The skirt can be lifted up by incoming fuel but downward pressure seals it against the cover surface.
The exit hole in the center of the valve incorporates a duckbill check valve. Fuel leaving the valve can push the valve's mouth open but pressure in the opposite direction presses the lips shut (#17).
The purge bulb seats into an annular groove in the pump cover and is retained by a flange and screws (#18).
Although it is commonly called a primer bulb, it really is a purge bulb. A primer feeds fuel into the carb throat and into the air intake stream of the engine providing a rich fuel to air ratio for easier starting. Purging circulates fuel through the carburetor to flush air and old fuel out of the carb's fuel pump and metering chamber and the fuel returns to the fuel tank.
The throttle plate is oriented as shown (#19).
Pump side of carburetor: Diaphragm (#20, D) is next to carb body, followed by the gasket (#20, E) and cover / purge bulb assembly (#20, F).
On the metering side of the carb: Gasket (#20, A) goes next to carb body, followed by diaphragm (#20, B) with the large stiffener plate and button (#20, H) facing the metering lever, and cover (#20, C). A small hole (#20, 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.