How-to decarbon a 2 or 4-stroke outboard motor

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Well-known member
Feb 16, 2006
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Lewes, DE
Probably one of the best tutorials I've read on the subject. ... 589&page=1


Member #7772
9023 posts
South Jersey
11:46 PM 6/19/2002

This works for Carbed, EFI, Ficht, HPDI, Optimax and even 4 strokes...
First you need a separate small fuel tank. One of those 3 gal red Tempos works great or an empty gal milk jug will also work, but might be a bit messier..

I use Seafoam over the OEM stuff like OMC Engine Tuner or Merc Power Tune because in the last few years they changed the formula and you have to let them sit up 12 hours. Who's got time for that?? Seafoam you can buy from NAPA, CarQuest or other auto stores. Seafoam works in 15 mins.

You'll need 3/4 gal of gasoline and one 16oz can of seafoam for each engine. Don't forget to add 3oz of oil if you are premixing in a carbed engine. Use about 3 ft piece of fuel hose off the little tank. You connect this tank to your engine by pulling off the main tank fuel hose off the intake side of your water separating fuel filter and plug the hose off the small tank on to that fitting. Or you can separate the fuel line on the tank side primer ball, so you can still use your primer. If you have an engine that has fuel plug then you need a fuel plug on the little tanks hose.

Start the engine, let it warm up and start pulling the mix into the engine. You may have to increase the idle to keep it running once she get loaded with the Seafoam. Run the engine 15 mins in the dock or just cruising around under 2500. Then shut it down and let it sit for 15 mins. Restart the engine, the smoke you see is the carbon burning off. Do the whole thing again and let her sit again for 15 mins. If she smokes after the second time do it again, but I've never seen one still smoke after three doses. The gallon mix should be enough to do this 3 times. You don't need any wide open throttle, you don't need to change the plugs. If it's cleaning the combustion chambers it's also cleaning the plugs, but every 50-60hrs is good time to change plugs in most engines.

I cleaned a antique evinrude one time that had a 1/4" of solid carbon on the exhaust chamber walls by running a 1/2 gal of the mix through it. Seafoam has been around since the 30's and it's what they used when they were burning straight 4 stroke 40SAE oils in outboards.

You guys with the 4 strokes think you are immune from this? Those engines work 10 times as hard as any auto engine ever will and they will carbon up. I bought a Bronco two years ago that had 95,000 miles on it. When I used seafoam on it I had the neighbors hanging out of their front doors looking for where the fire was after I started it the first time there so much smoke.

Too many are under the assumption that it's totally the 2 stroke oil that causes the carbon, Wrong... it's also the additives they put in the fuels today. The carbon inhibitors in 2 stroke oil are there for this reason also. Remember when gasoline used to smell like gasoline, today it smells more like bad cologne.

For those guys that like to do the carbon treatment by spraying it down the carbs Seafoam also comes in spray can called Deep Creep. It's the same stuff under pressure. Says right on the can Oxygen Sensor Safe, for you Yam guys.

After that if your engine maunf recommends a daily additive treatment then do that in the mean time, but all 2 stroke outboard need decarboned every 50-60hrs. If I owned a 4 stroke I would do it the same. Once you are set up with the tank and hose the Seafoam is only 5-6 bucks can. It to easy not to do it.

Modified by Dunk at 6:19 PM 8/1/2003