Should I Engage the Lawn Mower Deck at High Idle or Low Idle?

Power Take Off

• The power take off is a circular electrical unit that sits at the end of the clutch. This PTO clutch powers the cutting blades on the mower. Electrical wires connect the PTO to the mower blade switch, usually near the operating panel beside the steering wheel. When the operator presses this switch down, electrical resistance is stopped and the PTO starts sending power to the clutch, which in turn sends power to the cutting blades.

The Mower Deck

• The mower deck sits just underneath the drive train or the crankshaft of the engine. A system of belts and pulleys connect the crankshaft to the clutch. When the PTO is engaged, it allows the belts to tighten on the pulleys, allowing power to transfer from the engine to the cutting blades. On older model riding lawn mowers, the blades start turning simply when the engine starts and the operator doesn’t need to worry about engine speeds.

Low Idle Speed

• Most lawn mowers operate with a low and a high idle speed. The low idle speed, in general, is used for starting and stopping the lawn mower. At a lower speed, the engine is better equipped to start and stop the combustion process. This occurs in the carburetor. When the choke is opened, more gas fills the carburetor, allowing enough fuel into the cylinder for combustion to begin. When the operator goes to stop the engine, he engages the low idle speed to enrich the fuel mixture and slow down the combustion process.

High Idle Speed

• Most often, the high idle speed on a riding lawn mower with a PTO clutch is used for engaging and disengaging the blades and for all cutting purposes. When the mower’s speed gets set too high, the air fuel mixture in the carburetor gets leaner, with less fuel and more air. This allows the engine to combust the fuel at a greater rate and with more force, driving the crankshaft faster. This essentially gives the engine greater power. Adding an auxiliary function, like the belts, pulleys and blades, requires this extra power to prevent the engine from shutting down.