How to Adjust the Idle Circuit on a Lawn Mower Carb

The idle circuit on a lawn mower carburetor controls the idle speed of a lawn mower. Most lawn mowers idle around 1,200 to 2,000 rpm, and this minimum speed is required to sustain combustion. If the idle drops below this speed, the engine will shut off. If the idle goes above this speed, the engine may surge or run hot, potentially damaging the engine. When this happens or when the carburetor gets cleaned, the idle circuit will need adjusting.

Screwdriver
Tachometer

Instructions

- 1
  Turn the lawn mower's engine off to adjust the idle circuit. Unscrew the air filter box and remove the air filter box, which contains the air filter, from the side of the engine.
- 2
  Locate the idle speed mixture screw, located on one of the carburetor's side. Turn the screw slowly clockwise until the needle gently touches the seat. Turn the screw counterclockwise 1 ½ revolutions.
- 3
  Start the lawn mower's engine again and let it run for 10 minutes to achieve normal operating temperature. Connect the tachometer to the engine or hold it near the carburetor.
- 4
  Turn the idle speed screw clockwise or counterclockwise to set the idle speed at your lawn mower's idle specifications (See Owner's Manual).

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