Boat maintenance: Flushing outboards

By: Andrew Norton, Photography by: Trade-a-Boat

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  • Trade-A-Boat

Freshwater flushing is essential for all water-cooled outboards.

Boat Maintenance Flushing Outboards LEAD

Regular flushing helps keep the cooling water passages in top condition and reduces the possibility of your engine overheating.

Cooling water in the cylinder block and head absorbs heat from the combustion process and mixes with gases in the exhaust system to cool them and help scavenge the exhaust system. This is essential with carbie two-strokes as some of the air/fuel mix escapes with the burnt gases, and the better the scavenging the more efficient the combustion process.

The trouble is that when the engine is running at full load, such as at wide-open throttle, salt crystals separate from salt water and lodge in the cooling passages. In reality the crystals ain’t salt – they’re aluminium chloride formed from the reaction between salt water and the aluminium passages. Raw-water-cooled outboards are designed to run no hotter than about 65C to prevent the rapid accumulation of these crystals.

Crystal accumulation not only depends on how hot the engine is running but also the type of alloy used in its construction. US-made outboards such as Mercury and Evinrude/Johnson use either die casting or lost-foam casting that enables extremely low-copper alloy to be used. This alloy has a fine grain that doesn’t snag the aluminium chloride crystals.

However, Japanese outboards have traditionally used a high-copper alloy that is needed for easier pouring into sand castings. This alloy has a fairly rough surface that catches the crystals more easily.

Hence the old adage that taking a US outboard for a long run would flush the cooling system, almost eliminating the need for regular freshwater flushing. The same could not be said of Japanese engines where the cooling passages could clog up with crystals unless regularly flushed.

This clogging happens more rapidly in engines without a thermostat. To prevent the engine running too cold when trolling, the passages are smaller than they’d be were a thermostat fitted, and these small passages are more susceptible to clogging. A thermostat maintains running temperatures in a relatively narrow band so the cooling passages can be larger, allowing for a larger volume of water and more effective heat absorption from the cylinders. Controlling engine temperature is essential for efficient combustion and to meet emissions standards, which is why all water-cooled four-stroke outboards have thermostats.


Before flushing muffs became common when outboard manufacturers started locating cooling water intakes just above the gear case torpedo, the traditional way of flushing small outboards was to mount the outboard in a drum, then fill the drum with fresh water until the cooling water intake was well immersed. The engine would then be run for five minutes in the belief the crystals would be flushed out.

The trouble with this technique for carbie two-strokes is that they dump excess oil into the water, which is pumped up the cooling passages where it coats the walls and bonds to the alloy – particularly sand cast – reducing the effectiveness of the cooling system.

A smaller drum with overflowing water helps get rid of the oil and is really the only way of flushing outboards under 4hp. Few of these have thermostats so running them in gear at fast idle creates some engine load and heat to remove more crystals than if they were just idled in neutral.

Small four-stroke outboards use very little oil in comparison to carbie two-strokes so flushing in a drum is unlikely to give them the oil-coating issue.

You should never run a larger carbie two-stroke outboard in a drum as the much greater quantity of oil these push out can quickly coat the cooling passages and lead to engine overheating – as I found out with a Yamaha 15F I owned for eight years. I could have used muffs but thought the drum would be better. But oh, how wrong I was!

So with that lesson learned, the  Tohatsu M8B I’ve been testing for 12 years has always been flushed using muffs and it’s never had an overheating problem. In fact I’m still running the original water pump impeller, and cooling flow is as strong as it was when the engine was new.


These work well because only fresh water is pumped up the cooling passages – unused oil runs straight out the exhaust passages. However, flushing is more effective if the engine has a thermostat, as running a non-thermostat engine under no load never removes all the crystals. I find at least 10min at fast idle is needed to open the thermostat and flush crystals from around it and the cooling passages.

Outboards such as Tohatsus, Japanese-built Mercurys from 15hp upwards and Suzukis from 25hp upwards have auxiliary cooling-water intakes that need to be taped up before attaching the muffs, otherwise the impellers will suck air, run dry and be ruined. The twin-cylinder Evinrude and Johnson 5 plus the 6, 8 and old above-prop exhaust 25 have special attachments that clip around the intake ahead of the exhaust outlet.


Boat Maintenance Flushing Outboards 2

Amongst others, Suzuki and Tohatsu single-cylinder two and four-stroke outboards have provision for screwing in an attachment that connects to a garden hose. Simply tape up the main intake, turn on the water and flush the engine for around 10 minutes. However, with the two-strokes that don’t have thermostats it’s better to run them in gear in a flushing drum to raise engine temperature and flush out those unwanted aluminium chloride crystals.


Evinrude, Mercury and Tohatsu have flushing attachments that create a backwash around the thermostats and flush out the cooling passages without needing to run the engine. Not only does this save water and fuel but also avoids annoying your neighbours by running the engine late at night after a full day’s fishing. Unfortunately, apart from some smaller Evinrude, Mercury and Tohatsu four-strokes this feature is mostly limited to larger outboards.


Never run out of fuel while flushing a carbie two-stroke – the lack of fuel also means no oil, so the bearings could be damaged and cylinder walls scuffed. Way better to use a fuel stabiliser during flushing, such as Stabil or Evinrude/Johnson 2+4 Fuel Conditioner, and stop the engine before the carbie runs dry.


Evinrude and Johnson:

  • Carbie two-stroke 1.5, Colt, Junior, 2.3 and 3.3, 4 and 5 (single-cylinder).
  • The 5 should be flushed in reverse at fast idle to load the engine but avoid ejecting water from the drum.
  • Evinrude four-stroke 3.5, if not using the backwash attachment.


  • BF5. This should be flushed in reverse at fast idle.

Mariner and Mercury:

  • Carbie two-stroke Thruster, 2, 2.2, 2.5, 3.3, 4 and 5.
  • The 4 and 5 should be flushed in reverse at fast idle.
  • Four-stroke F2.5 and F3.5, if not using the backwash attachment.


  • Carbie two-stroke DT2, DT2.2, DT3.5, DT4 and DT5 (single and twin-cylinder).
  • Both DT5 models should be flushed in reverse at fast idle.
  • Four-stroke DF2.5.


  • Carbie two-stroke M2.5, M3.5A, M3.5B, M4 and M5B.
  • The M4 and M5B should be flushed in reverse at fast idle.
  • Four-stroke MFS2.5 and MFS3.5 if not using the backwash attachment.


  • Carbie two-stroke 2B, 2C and 3A.
  • Four-stroke F2.5A. 

See the full version of this review in Trade-A-Boat #495. Why not subscribe today? 


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