How to Adjust the Idle Speed on a Murray Lawn Tractor

Float-Type Carburetor

• 1

  Locate the idle speed screw, adjacent to the air filter and cartridge, if you have a float-type carburetor. This screw keeps the throttle plate (inside the carburetor) from closing completely. Find the idle mixture screw that regulates the fuel flow at idle speed.

• 2

  Turn off the engine and remove the air filter and air cartridge.

• 3

  Turn the idle mixture screw clockwise, until the needle lightly touches the seat. Then, turn it counterclockwise 1 1/2 turns. Replace the air filter and cartridge and start the engine. Make final adjustments with the idle speed screw.

With Float Bowl

• 4

  Turn the main jet adjustment screw, at the base of the float bowl, clockwise, if your carburetor has a float bowl. Turn it until you feel it just reach the seat inside the emulsion tube. Then make 1 to 1 1/2 counterclockwise turns.

• 5

  Bring the engine to standard operating temperature by running it for a few minutes. Turn the idle mixture screw clockwise to slow the engine revs. Use a tachometer to judge the engine speed. Set the revs at 1750 rpm for an aluminum-sleeve engine and 1200 for a cast iron engine.

• 6

  Keep the engine idling while you hold the throttle lever against the idle speed screw to produce what the Briggs & Stratton website calls "true idle." Make adjustments with the idle mixture screw for fine tuning.