How to Adjust the Gas Intake and Speed on a Johnson Outboard Motor

by Will Charpentier

When you adjust the gas intake and speed of a Johnson outboard motor, you're accomplishing two tasks in one. You're setting the motor's low speed, it's idle speed, and you're adjusting the mixture so that the motor performs at its optimum efficiency at both high and low speeds without sputtering or coughing. You might find, after the adjustment is complete, that the motor runs a little faster at idle, and at full speed it may no longer sound as if it's surging. You'll also find that more of your gas money stays in your wallet.

Push the adjustment knobs that came with the motor onto the low speed and high speed mixture needles so they can be turned easily during this procedure. Install the low speed knob with the pointer up and the high speed knob with the pointer down. These are temporary installations and the knobs can be removed after the adjustments are completed.

Launch your boat. Start the motor and allow the engine to warm up normally. During warm up, run the engine at or a little more than about half throttle. During warm up, take this opportunity to inspect the motor for potential fuel leaks. Have an assistant hold the wheel while you make the adjustments.

Once the engine is fully warmed, run the motor at full throttle and carefully adjust the high speed needle until the motor runs at the highest consistent rpm. Slow the engine to an idle speed of about 700 rpm or 800 rpm and adjust the idle speed needle until the motor runs at the highest consistent idle rpm. Turn the needle in small increments, allowing at least 15 seconds for the idle speed to stabilize before making further adjustments.

After the final low speed adjustment is made, turn the low speed needle out counterclockwise 1/8 of a turn to make sure it is not too lean at idle speeds. When the low speed needle is set, go back and re-adjust the high speed needle. After the final high speed adjustment is made, turn the high speed needle out counterclockwise 1/8 of a turn to make sure it is not too lean above idle speeds.

Remove the knobs. Hold each needle steady and tighten the packing nut to lock the needle into its new position.

Items you will need

  • Assistant

Tip

  • If you can't find your knobs, your Johnson dealer can provide replacements.

References

  • "Johnson Repair Manual 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.