OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters Mcculloch trim mac 240

I'll try my best to explain it:

A 2 stroke carburetor (zama, walbro etc) has the same principle of a 4 stroke carburetor but uses different methods to draw fuel into engine. I'll try to explain its different parts.

Fuel pump diaphram:
This carburetor uses vacuum and pressure pulses from the crankcase that moves the fuel pump diaphram up and down. When the pump diaphram moves up it expands the area in the pump chamber and creates a vacuum and sucks fuel into the chamber through a check valve. When the pump diaphram moves down it compresses the area in the pump chamber and fuel is then forced through a check valve into the inlet needle.

Metering diaphram:
The normal float and needle and bowl system on a 4 stroke carburetor is the same principle here. On a two stroke carburetor you cannot use a float and bowl because you need to rotate the machine on extreme angles, which would cause the float to fall and shut off the fuel.
On this carburetor it uses a needle, metering lever, spring and metering diaphram. The spring closes the needle which is connected to the metering lever and keeps fuel from entering the chamber. As the engine runs fuel is being drawn from the metering chamber and into the carburetor. This makes the metering diaphram push against the metering lever. The fuel pressure from the pump is great enough to overcome the spring pressure, and thus opens the needle which lets fuel into the chamber.

Hope that explains the roles of the diaphrams in the carburetor. As for the primer, it has a major role in getting fuel into the carburetor for cold starts. If you ever take a primer bulb off, you will notice two check valves, one inlet and one outlet. When you press the bulb, the outlet check valve opens which forces fuel/air into the fuel tank. When you release the bulb, the outlets check valve closes, and a vacuum is created. This vacuum that is created in the metering chamber, pulls the metering diaphram onto the lever and pulls the needle off its seat. The open needle now causes a vacuum to draw the fuel from the fuel tank, through the carburetor and back to the primer bulb.