The idle speed on Evinrude 40-horsepower outboard motors is controlled electronically through the idle air control (IAC) valve. The valve supplies large amounts of air to the engine to produce a fast idle for engine warmup. Once the engine reaches normal operating temperature, the valve closes and all idle air is supplied through the IAC bypass. Perform the warm engine idle speed and bypass air screw check and adjustment annually or during every other tuneup.
- Shop tachometer
- Garden hose
- Motor flushing attachment
- Plastic IAC valve test plug
- Duct tape
- Replacement IAC valve
Connect a shop tachometer to the motor, following the tachometer manufacturer's instructions. Connect one end of a garden hose to a water source and the other to a motor flushing attachment. Clamp the flushing attachment's "earmuffs," so-called because they resemble that piece of winter apparel, over the raw water intakes on the sides of the motor's lower unit and turn the water supply on full force to provide cooling water to the motor.
Move the gear selector to "Neutral." Start the motor and allow it to idle until it reaches normal operating temperature. Set the throttle to "Idle."
Unplug the IAC valve hose from the air silencer. Block airflow into the hose with a plastic IAC valve test plug or piece of duct tape. Remove the rubber plug from the idle speed screw opening just below the air intake silencer overhang.
Slowly turn the idle speed screw with a screwdriver until the engine reaches 800 rpm. Turning the screw clockwise to decrease airflow and engine speed. Turn the screw counterclockwise to increase air flow and engine speed.
Push the throttle into "Forward" until the shop tachometer reads 2000 rpm, then slowly return it to the idle speed. Allow a few minutes for the motor speed to stabilize, then readjust the idle speed to 800 rpm.
Remove the plug or tape from the IAC valve hose, shift the engine into "Forward" and note the idle speed. If idle speed is greater than 825 rpm or less than 725 rpm, replace the IAC valve. Shift the engine to "Neutral" and shut down the motor.
Remove the shop tachometer and reinstall the rubber plug on the idle speed screw. Turn off the water supply, remove the flushing attachment from the motor and the garden hose, and store both the attachment and the hose.
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