How to Adjust the Carburetor on a 40-HP Mercury 2-Stroke Outboard

How to Adjust the Carburetor on a 40-HP Mercury 2-Stroke Outboard

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The parameters for adjusting the carburetor on a Mercury outboard involve both the idle speed screw that controls the motor's speed at idle, and the mixture screw that controls the fuel-to-air ratio. Adjusting both of these requires that you have the motor running. While the adjustments seem minor, both affect the performance and economy of your motor.

Items you will need

  • Adjustable wrench

  • Screwdriver

  • Machinist's ruler

  • Flushing attachment

  • Garden hose

  • Shop tachometer

  • Pliers

Move the motor's shifter into "Neutral." Loosen the throttle cam follower screw. Turn the idle speed screw until the throttle plate positioner does not touch the idle speed screw. The throttle plate will close.

Loosen the jam nuts, located right below the throttle cam on the throttle cable. Move the the throttle to the idle position. Set the cam follower roller against the throttle cam. Adjust the throttle cable sleeves in their mounting bracket so the roller is centered on the raised mark on the throttle cam, while allowing some slack, between 1/16 inch and 1/8 inch in the throttle cables, as measured with a machinist's ruler.

Tighten the jam nuts on the throttle cable and tighten the cam follower screw. Turn the idle speed screw clockwise -- to the right -- until there is a gap of between 0.005 inch and 0.040 inch between the throttle cam and the cam follower.

Attach a flushing attachment to a garden hose. Set the flushing attachment over the raw water intake ports of the motor and turn the hose on full force. Start the motor and warm it to its normal operating temperature range.

Attach a shop tachometer to the motor's Number 1 cylinder. Turn the idle speed screw on the carburetor linkage to adjust the motor's speed to between 700 and 800 rpm.

Put the motor in "Forward." Turn the mixture screw clockwise until the motor begins to misfire. Turn the screw counterclockwise one-quarter turn or until the motor no longer misfires. With the motor in gear, "punch" the throttle for sudden acceleration and readjust the mixture screw if it "bogs down" before accelerating. Recheck and readjust the idle to between 700 and 800 rpm.


  • If you work on your outboard motor when your boat is on its trailer, or your motor is on a storage stand, bend the tabs of the retaining washer away from the propeller retaining nut with a pair of pliers and remove the propeller nut from the shaft. Slide the propeller, the washer, spacer and thrust washer from the propeller shaft. Failure to remove a propeller before operating an outboard out of the water during maintenance or long-term storage is an invitation to a propeller-strike injury, which can maim or kill.


  • When you measure the slack in the throttle cable, rock the throttle cam from side to side, measuring the amount of travel between the throttle cam and the link rod ball.
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