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. 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. 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. 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.Ī smaller drum with overflowing water helps get rid of the oil and is really the only way of flushing outboards under 4hp. The engine would then be run for five minutes in the belief the crystals would be flushed out. THE 44-GALLON DRUMīefore 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. Controlling engine temperature is essential for efficient combustion and to meet emissions standards, which is why all water-cooled four-stroke outboards have thermostats. 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. 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. This clogging happens more rapidly in engines without a thermostat. The same could not be said of Japanese engines where the cooling passages could clog up with crystals unless regularly flushed.
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. This alloy has a fairly rough surface that catches the crystals more easily. However, Japanese outboards have traditionally used a high-copper alloy that is needed for easier pouring into sand castings. This alloy has a fine grain that doesn’t snag the aluminium chloride crystals. 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. Crystal accumulation not only depends on how hot the engine is running but also the type of alloy used in its construction.