How to Decarb a Boat Outboard Motor

Decarb boat outboard motor

Decarb boat outboard motorIt might be time to decarb your boat motor if you start it and find that white smoke starts to billow out of it. Decarbing is short for “decarbonization,” a term that refers to decreasing the amount of carbon in metal. During the course of time, carbon tends to build up along the exhaust chamber walls of a boat motor. This causes problems such as odors, smoke, and rough engine or idle performance. But that doesn’t mean that you dock your boat and wish you had another one.

Decarbing 2 Stroke vs 4 Stroke Motors

Decarbing is generally not necessary on 4-stroke motors as they don’t build up nearly as much carbon as 2-stroke motors do. However, if you spend a lot of time at idle or trolling and rarely push your motor to the upper RPM ranges, you may wish to occasionally decarb as a preventative measure.

Here are two affordable and timely ways you can decarb your boat motor:

Decarb Method 1:

What you will need:


  1. First, disconnect the primary gas tank from your boat’s motor.
  2. In a spare tank of boat gas, mix around 3/4 gallons of fuel with a 16-ounce can of motor treatment.
  3. Connect the secondary tank to the fuel intake port of your boat.
  4. Start the engine up and give it a few minutes to warm up.
  5. On the secondary tank, squeeze the primer ball and allow the motor to pull in the motor treatment.
  6. Keep the engine running for about 20 minutes.
  7. Stop the engine and allow it to sit for another 20 minutes.
  8. Start the engine up again. If you find that there is no smoke billowing out of it, you have successfully removed the carbon buildup in your engine. Repeat the process if you still find a little smoke billowing.

Decarb Method 2:

If the first method doesn’t work, try this:

  1. Pick up a few cans of engine decarbonizer. Make sure you use a known brand.
  2. Pull out the plugs from your engine and spray the engine decarbonizer into each cylinder.
  3. Next, spin the engine over a couple of times and repeat the process to ensure that it has seeped into the rings and top of pistons.
  4. Store the outboard in a fully upright position and allow it to sit overnight.
  5. The following day, crank your engine up. Don’t be alarmed with the smoke that is still billowing, as it is the carbon that is leaving your engine through your exhaust.
  6. Remove your intake cover and cowling so that you can reach your carb throats with the engine decarbonizer.
  7. With the engine at full throttle, spray the engine decarbonizer into each carb throat.
  8. If the engine dies, crank it right back up and repeat process once more until you use one more can.
  9. Wait for about an hour and a half. While you are waiting, remove the plugs, and wipe it down so as to remove all the carbon pieces that have gotten stuck.
  10. After the stipulated time, start the engine up and run it full throttle until the smoke starts to clear.
  11. Replace your plugs when you are all done.

What Decarbing A Motor Looks Like

Here’s an example video of what it looks like when you decarb a boat motor. Lots of smoke!


Have any tricks to share?

If you have any tricks on how you decarb your boat motor, please reply in the comments. We’d love to hear about them!

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