Mcculloch Mac 140 Chainsaw Manual

Suitable Replacement McCulloch MAC 140 Carburetor Kit

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McCulloch MAC 140 Carburetor Kit - General Carb Kit Info

The carburetor has two different sides. First, the fuel pump side of the carburetor pumps fuel from the fuel tank through the fuel line to the metering side of the carburetor. Second, the metering side of the carburetor supplies fuel to the jets in the carburetor so the saw idles properly. Additionally, this side allows the saw to accelerate to full throttle instantly when the throttle is activated [pulled]. Finally, this side maintains fuel to the chain saw motor at full throttle regardless of the position of the saw.

The McCulloch MAC 140 carburetor has diaphragms and gaskets to help it pump and meter the fuel by crankcase impulse. Every time the piston goes up and down, it creates vacuum and pressure in the crankcase. The carburetor uses this pulse via a pulse line connected to the crankcase or through the carburetor intake to get the pulse needed to properly make the carburetor function.

On the metering lever side of the McCulloch MAC 140 carburetor, there is a diaphragm and gasket with a cover plate screwed to the carburetor. The metering diaphragm controls the amount of fuel allowed to enter the metering lever side of the carburetor. The diaphragm is either connected or just touching the fulcrum [meter lever] and when fuel exits the metering side of the carburetor through the jets in the carburetor, atmosphere pressure [14.7 psi] pushes the diaphragm in letting more fuel in. As the fuel rises to fill the metering lever side of the McCulloch MAC 140 carburetor, the inlet needle valve closes the fuel off from the fuel pump side of the carburetor. The needle valve goes into the body of the carburetor and is fastened to the metering lever fulcrum which has a spring under the opposite side of the needle valve to keep pressure on the needle valve when closed.

On the metering lever side of the McCulloch MAC 140 chainsaw carburetor there are two sides, a low side and a high side. The low side controls idle and acceleration when the saw starts to rev up. The high side controls the amount of fuel the saw will receive at full RPM. Some carburetors will have 2 adjusting screws, one for the low side and one for the high side. There is also an idle screw which makes contact with the throttle shaft to control idle speed along with the low side of the carburetor. Note that some carburetors have just one adjusting screw and the other side is prefixed and non-adjustable. Some may also have no adjusting screws [except the idle screw] to preset or pre-jet the carburetor.