• Quick notes:
• For those carbs that have adjustable idle and/or main fuel mixture screws the preset (preliminary adjustment) for the needle valve is 1-1/2 turns out from lightly seated.

'Opposed twin-cylinder' refers to the fact that the two cylinders are 180° opposite each other, as compared to a V-twin configuration. These engines use a float type down-draft two-piece Flo-jet carburetor with a built-in fuel pump (#1). The float and choke are part of the upper body of the carb. The high speed and idle fuel circuits are separate. Only the idle speed is adjustable. The high and low speed fuel mixtures are set by fixed (non-adjustable) jets.

Four mounting screws are used on later versions of this carb (#2). Earlier versions had three screws. The pump is operated by the pulsations of crankcase vacuum fed into the pulse tube nipple (#2, blue) as the engine runs. Fuel enters the pump at the nipple shown at #2, green. There are two vent holes in the cover (#2, red) that must not be blocked by dirt or insect activity.

Behind the pump cover plate is a gasket and then a diaphragm (#3). Ensure that the diaphragm is not perforated, deformed, brittle or gummy.

After the gasket and diaphragm is the pump body. Fuel enters the body at the nipple (#4, pink arrow) and flows through the lower check valve (#4, blue). Fuel returns through the upper check valve (#4, red) from the main reservoir on the other side of the pump body and exits to the carb via the passage at #4, green. Check that the check valves are free to move, and free of foreign matter and that they close fully. They are not meant to be disassembled from the body.

The main pump diaphragm (#5) is between the pump body and the carburetor. Ensure that the diaphragm is not perforated, deformed, brittle or gummy.

Behind the main diaphragm there is a spring and a cup (#6) that sits on top of the spring. The cup protects the diaphragm from damage by the spring. Vacuum pulsations from the crankcase arrive at the carb via the pulse tube nipple (#6, yellow). These pulses work against the spring pressure causing the diaphragm to pump back and forth, drawing fuel from the lower check valve and pushing it out the upper one. Fuel then exits the pump body and enters the carb at (#6, pink).

There is a small vent at the bottom of the vacuum chamber (#7) that should not be obstructed.

Older carbs used a 3-screw pump and had an adjustable idle mixture screw (#8, green). The preliminary setting for the idle mixture screw is 1-1/2 turns out from lightly seated. There are two vent holes in the pump cover (#8, pink).

The 3-screw fuel pump has three springs. There are two valve springs that sit on bosses (#9, pink) and one diaphragm spring on which a cup sits (#9, green). The cup protects the diaphragm from damage by the spring. Fuel enters the pump at the side nipple (#9, red) and vacuum pulses arrive at the bottom nipple (#9, yellow).

Fuel enters the carb from the pump by means of a passage through the casting (#10, yellow).

After removing four screws, the upper body of the carburetor lifts straight up away from the lower body. The float, inlet needle, gasket, and main emulsion tube go with the upper body (#11). Fuel that traveled from the pump, through a passage in the lower body, arrives at the upper carb body at (#11, red).

With the carb upper body upside down the float should sit parallel to the body. If it does not, the float may be stuck or deformed, the rubber tip of the inlet needle may be damaged, or there may be debris between the needle and seat. To remove the float, push the float hinge rod out from one side and pull it out with needle-nose pliers (#12). The float can now be lifted up with the inlet needle attached.

When the float is lifted out, the inlet needle will come out with it. It is assembled as shown (#13). Inspect the needle's rubber tip for wear, corrosion or damage. Shake the float to see if fuel has leaked into it.

Fuel from the lower body arrives at the upper body at #14, red and continues to the inlet needle seat (#14, blue) through a passage in the casting. Fuel is allowed to pass the inlet valve into the fuel bowl until the fuel level rises sufficiently to cause the float to rise, pushing the inlet needle against the seat.