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

The Zama carburetor body type number is embossed on the carb body (#1). This is a C1U body type. Other body types include C1, C1M, C1Q, C1S, C1T, C3, C3A, C3M, G1L, LA, LB, LE, RB, RC1, and RC2. The 'lo' and 'hi' fuel mixture adjusting screw locations are labeled 'L' and 'H' respectively. The 'lo' screw is always closer to the engine.

The Zama carburetor model number is stamped on the carb body (#2). This is a C1U M29A model.

The throttle adjusting screw (TAS) or idle speed screw (#3) is incorporated into the pump cover. A small plastic friction piece (#3, A) is inserted into a pocket drilled into the threads and keeps the TAS secure.

The 'lo' and 'hi' fuel mixture adjusting needle screws (#4) are labeled 'L' and 'H' respectively. The 'lo' or idle screw is nearer the engine. A good initial setting for both screws is 1 turn out from lightly seated.

The fuel mixture screws are are not interchangeable (#5). The 'lo' needle is longer and has a finer thread. Check the tips for damage. A deformed tip will make the carb difficult or impossible to adjust. The springs keep the screws securely in position.

The pump cover and metering chamber cover both have alignment pins (#6,yellow) so they can be installed only one way.

The pump diaphragm (#6,C) goes next to the carb body, followed by the gasket (#6,B) and then the pump cover (#6,A). When replacing the pump diaphragm, use the clear or shiny black diaphragm in the kit if the carb surface is smooth, otherwise use the rubber diaphragm..

The metering chamber gasket (#6,F) goes next to the body, followed by the diaphragm (#6,E) and then the cover (#6,D).

A spring and steel ball are set into a pocket behind the choke shaft (#7). The ball rides in a groove cut into the shaft and two dimples serve to hold the choke in two positions: half open and full open.

Two small flaps (#8, J) on the diaphragm act as check valves. Fuel moving up from the carb body can push the flaps up but cannot travel in the other direction. Fuel is drawn into the carb at the inlet nipple (#8, A) and pushes past the diaphragm flappper valve (#8, B). From there the fuel enters a space in the cover (#8, C), travels to the pump chamber (#8, D) above the diaphragm. This area of the diaphragm (#8, K) is responsible for the pumping action as the diaphragm moves up and down in response to pressure pulses from the crankcase acting on the opposite side of the diaphragm. The pulses arrive via a drilled hole in the body (#3, B) and reach the diaphragm at (#8, L). Fuel exits the pump chamber via a channel and leaves the pump cover (#8, E), entering the carb body. At (#8, F) the fuel flows through a drilled passage, pushes past a second flapper valve (#8, G), enters a space in the gasket (#8, H), passes through the fuel inlet screen (#9) and reaches the back side of the inlet needle seat (#8, I) seen at the bottom of the hole.

Fuel leaves the pump side of the carburetor through a fine wire mesh filter screen (#9, blue) on its way to the metering valve. In addition to collecting any fine solids or gum, once this screen has been wetted with fuel/oil mix, water will not pass through it and a single drop of water will block fuel flow. This screen is the most often clogged part of the carb.

Fuel that has passed through the filter screen travels to the metering valve (#10, A) on the other side of the carburetor. Also shown here are the low speed circuit welch plug (#10, B) and the main fuel nozzle (#10, C).

The rubber tip of the metering valve needle (#11, A) seals against the valve seat (#14, A). The metering lever (#11, B) engages the needle and pivots on a steel pin (#11, C). A spring (#11, D) sits under a dimple (#11, E) in the lever, pushing the needle tight to the seat. A single screw secures the assembly.

The free end of the metering lever is adjusted to be 0-0.012in. (0-0.3mm) below the surface of the carburetor body by bending it.

The main fuel nozzle (#12) incorporates a check valve to prevent air from being drawn into the metering chamber during low speed operation or during purging. The check valve is simply a small plastic disc, a little larger than the low speed check valve, that sits atop the nozzle orifice. It is held in place by a cage which allows clearance for the disc to lift up from the orifice as fuel flows into the venturi. The top of the nozzle is formed over to secure the cage. The nozzle is pressed into position and can be replaced.

The idle circuit welch plug (#13, A) is an oblong convex aluminum part which when set into the idle well cavity and flattened with a punch seals the cavity. Behind it there is a check valve (#13, C) which seats on a flat shoulder (#14, D) machined into the carb body. Between the valve disc and the welch plug there is a plastic retainer (#13, B) that gives the disc clearance to lift off the seat and allows fuel to flow to the idle fuel discharge ports (#14, F).

The fuel inlet valve seat (#14, A) can be seen after the valve assembly has been removed . With the idle well welch plug removed, the idle check valve seat (#14, D) and the idle fuel discharge ports (#14, F) are visible. The main fuel passage (#14, B) from the main fuel pickup port (#14, C) to the main nozzle can be seen once the nozzle is pressed out.

Fuel that has passed into the metering chamber from the inlet valve can travel to three possible destinations. The first possibility is that fuel enters the main fuel pickup port (#14, C), passes the high speed fuel mixture adjusting screw and seat, travels through a drilled hole (#14, B) where it enters the main nozzle. It then passes the main check valve and enters the venturi airstream.

The second possible route takes the fuel from the metering chamber to the idle fuel pickup port (#14, E), past the low speed fuel adjustment screw, past the idle check valve (#14, D) and finally to the idle fuel discharge ports (#14, F). While fuel is being drawn through the idle discharge ports into the engine intake airstream the check valve in the main nozzle prevents air from being pulled into the metering chamber.

On another possible path fuel enters a drilled hole (#14, G) that connects to the outlet nipple (#14, H) to the remote primer bulb. As the primer is operated, the check valves in the idle and main fuel circuits prevent air from being drawn into the metering chamber.

As fuel exits the metering chamber by any of these routes, atmospheric pressure moves the metering diaphragm (#6, E) toward the carb body. When this pressure, in concert with the pressure supplied by the pump, is sufficient to overcome the spring (#11, D) the button (#6, H) on the diaphragm presses against the central end of the metering lever (#11, B), which pivots on its fulcrum (#11, C), lifts the needle (#11, A) off its seat, allowing fuel to flow from the pump side of the carb to the metering chamber.

The check valve (#15) for the idle circuit is a small thin colorless transparent plastic disc that is virtually invisible except under just the right lighting. The check valve in the main nozzle is slightly larger in diameter.

Fuel destined for the main nozzle enters a port (#16, A) in the metering chamber, passes the main fuel needle screw (#16, G), continues down a drilled hole (#16, B) to the main nozzle, which has been removed in this photo (see #10, C).

Fuel for idle circuit enters a port (#16, D), passes the idle fuel needle screw (#16, H) , continues down a drilled hole (#16, Pointer) to the idle check valve (#16, E), which has been removed in this photo, to the idle fuel discharge ports (#16, F).

The idle fuel discharge ports deliver fuel from the idle fuel well (#14, F) into the engine intake airstream. The primary discharge port (#17, A) sits just past the closed throttle position and delivers fuel when the engine runs at low idle (throttle nearly fully closed). The secondary discharge port is located prior to the throttle plate (#17, B), delivering fuel at intermediate idle speed (partly open throttle). In this photo the throttle plate has been removed for better visibility of the ports.