Flushing Outboard Motors

Flushing outboard motors seems to be a good thing, depending on the water where you fish.  Fresh water fishermen are pretty lucky, but saltwater fishermen, if they don’t flush the motor soon after usage may encounter long term problems of salt buildup.  Also having the ability to run a motor without taking the boat to the lake is a plus, especially if you have done some repair work on it and need to see if your efforts were a success or tune the carburetor.  Or as I do, after any extended period of not being on the water, the evening before new fishing trip, which has saved the day numerous times.

Now depending on the year, the make and size of motor, the flushing may have to be done differently.

Most of the early motors may not have had any flushing provisions.  Then in the mid 1960s OMC and others needed a adapter kit to put water up into the pickup tube BEHIND the prop.  After about 1974, you could use a rubber flushing muffs.  Then about 1993 OMC put a 3/8″ plug on the water jacket to allow a flushing adapter to be screwed into to flush the motor.  Or if your motor is small enough, you can place it in a 30 gallon garbage can or 55 gallon barrel.

Water Flushing: These mid 60s up through 1973, most of the OMC motors main water supply to the water pump comes from a screened tube right behind the prop in the exhaust outlet as seen in the LH photo below.  Flush kits for this motor are rather scarce as Tempo has discontinued them.   Shown below is a new unused Tempo kit purchased off eBay.

Top view Bottom view


Foam lined plate with a opening to accommodate the screened pickup tube Instructions
Click on thumbnail below for larger image

There is also hope in another aftermarket, or Do It Yourself item, as seen in the center photo below, the LH (port side) of the exhaust housing just above the anti-cavitation plate there thin stainless steel plate held in by 2 screws.  In this plate are 4 small scoops that can help put water into the water pump area, as shown in the photo on the left below.  This is common to may different size of these motors in these years up to about 1974.  It was designed to supply water to the water pump when the motor was in reverse.

The right photo below is from a 10hp is an commercial aftermarket adapter (no longer available) with a 3/8″ course thread placed inward in the rearward part.  This allows the usage of Mercury’s Quick-Silver Flushing Device described below.  This flush plate apparently was NOT designed to be used when running, only for flushing, however this motor was a running motor, with no evidence of failure.   Since these are no longer made, you can fashion one by using the original plate as a sample and soldering/brazing, (even epoxy) a 3/8″ course nut onto the outside of it to screw the flushing adapter into.  Or if your intention is to leave it in place and run the motor that way, I would solder the nut on the inside to give less resistance and less chance of debris hanging up on anything exposed.

The center photo below shows this plate on the motor.  These have 4 small scoop holes and the palate is called a reverse water intake.  These are placed so that with these motors that use the screened intake tube behind the prop as seen by the red arrow on the LH photo below, when in reverse the prop is not forcing water into the intake.  These scoops provide water to bypass and enter the water pump inlet chamber.

I have not been able to verify that these scoops should be pointing forward or rearward.   However I have since talked to a old time marine mechanic about the direction of those holes.  Either he did not understand what I was saying or since his son took over the business and he has been out for a number of years, he forgot or did not pay any attention at the time.  After looking at as many as I can come onto, I now have concluded that the scoops probably were originally installed forward.  A few of these have this plates were painted over originally and little evidence that it had ever been off.  Plus if you look at a factory exposed drawing, these arrear to be on the forward end of the plate.AND thinking about it, this would supplement a partially plugged intake screen behind the prop, which seems common with these motors.

The thought of them facing rearward IF they are indeed a reverse intake, sounds plausible, however when in reverse, you would at minimal speed and will not be doing this for a extended period of time, so possibly either way would suffice. 

The water pickup tube on a Evinrude 6hp Fisherman  Evinrude with original plate on the 6hp Fisherman made up to 1979 Johnson with aftermarket flush plate

Mercury sells a flushing adapter that uses a 3/8″ National Course bolt thread on one end and a female garden hose end on the other.   Mercury’s Quick-Silver part number is  #24789A 1 with the actual name being “Flushing Device”.  Also OMC Johnson/Evinrude uses the same adapter to back-flush their 9.9/15hp 2 stroke outboard motors (and maybe more models) made from 1993 to 2007.   These adapters also fit about all of the Japanese outboards that use a back-flush system.

To flush, exchange the existing plate, replace it with the flush plate, screw the garden hose into the adapter, then into the motor, then turn on the water.  It does not need to be full force of the standard house water pressure, but about 1/2 force.   Start the motor up and let it run for long enough to get the motor warm enough to open the thermostat allowing water to flush thru the power-head.  

Mercury Outboard Flush Device  #24789A 1

OMC made after 1993 have a flush plug on the side of the water jacket as shown on the 1994 9.9hp motor.   Removing this plug screw allows you to use the above flush adapter.   Yamaha also for many years used this same system using the same adapter. 

These plugs are not designed to actually run the motor when using them, but to back-flush from a non-running motor, which is better than not at all.

Here is the flush plug on a 1994 Johnson 9.9hp

Yamaha has a slightly different method, by having a plastic tube with a garden hose fitting that is screwed onto the motor.  When you want to flush, unscrew this fitting, attach a water garden hose to it.   This back flushes thru the overboard water tell-tale system.  There is water to it all the time and is just blocked by being attached to the “plug on the motor”.

Shown here is the flush hose on a Yamaha T8

Muff Flushing: This is more common for motors of many sizes made after about 1980, where the water intake screens are mounted on the sides of the lower unit and below the water pump.  When attached to a garden hose, this rubber muff straddles the lower unit of the outboard motor, supplying water into the same location where the motor would pull it when mounted on a boat.

Shown here is the muff flushing attachment on a 1998 long shaft 9.9hp Johnson Here is a closer view of the photo on the left

Barrel Flushing: Then there is the old standby, a 55 gallon barrel.   These can be used for motors up to about 20hp.  You can use a garbage can but don’t try to put it in gear.

In building this test barrel, I found that if you do not cut the whole end out, but leave a portion at the rear that can be turned down, this allows you to even momentarily test the motor for a higher RPM without blowing a majority of the water up out the back.

Note the fitted wooded mounting bracket.

Using this rear baffle in the barrel, a higher RPM can be achieved.


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