McCulloch 160 Chainsaw Carburetor Specifications

The Carburetor

• The carburetor used in the 160, 100 series and the Eager Beaver line of chain saws will either be the Zama M1-M7 or the Walbro MDC 16. These carburetors are nearly identical in scope and specifications. Both use two needles to control the high and low speeds of the engine, and both have an integrated fuel pump that uses a diaphragm to pump fuel into the carburetor. Because these engines are more than 20 years old, the operator will need to disassemble the carburetor and check that the carburetor being used corresponds to either the Walbro or Zama carburetor.

The Idle Needle

• The idle needle regulates the flow of gasoline coming into the carburetor when the engine is at idle speed. This screw leaves open an inlet valve, allowing just enough fuel in to keep the engine combusting. If this screw closes too much, the engine won’t idle. If the screw is set too wide, too much gasoline will enter and the chain saw won’t accelerate to high speeds properly. The specifications for this needle will vary with the age and condition of the carburetor.

The High Needle

• The high speed needle regulates the flow of gasoline coming into the carburetor when the engine is accelerating and when it reaches its wide-open-throttle position. This screw controls the main intake jets on the carburetor’s mixing chamber. If the screw is opened too wide, too much fuel will pour in and the engine will not reach top speed and might drop its load under power. If the jet is closed too much, too little fuel will reach the engine, and the combustion will get too hot, and could blow out a piston seal. Again, the specifications will vary with age and condition of the carburetor.

Setting the Needles

• With older carburetors, the needles will need to be set manually and by ear. This poses a greater challenge to the operator because the room for error increases, and a blown-out seal could ruin an engine. Only make minor adjustments to these screws, in 1/8 increments. The ideal idle speed will be set just before the clutch engages and the chain starts spinning. The idle speed will also allow for a smooth transfer from low to high speeds. The ideal high speed will provide a quick acceleration to wide-open-throttle position. The engine will sound clean, but not high-pitched, whiny, or bubbly when it reaches the open position. Never adjust the screws beyond their stop positions.