How to Test Run an Outboard Moto

One thing commonly known to most boaters, but not all newbies, is that most outboard motors are water cooled (either artificially, or on a boat in a lake) and the water is pumped up into the powerhead to cool it as these have no radiators to cool the engine like cars do. This being the case, NEVER run the motor without it being supplied sufficient water in one manner or another. Otherwise you will (1) ruin the water pump impeller even by running it a few seconds. (2) Or if ran longer, since the motor is not being cooled, it will get HOT enough to loose the temper in the piston rings, possibly even scoring the cylinders and seizing the pistons in the cylinders which will lock up the motor. THIS IS NOT A GOOD THING. Usually this is also accompanied with the powerhead becoming so hot that the paint burns. A white powerhead now will be brown in the piston/cylinder area. And it will be expensive to repair, IF EVEN POSSIBLE depending on the age of the motor and/or parts being obsolete.

Therefore, you need to be informed on how to test run your motor when not on a boat, in the water. This could be after you take it out of storage (long or short term), or if it was used in saltwater, it is then best to flush (run) it with freshwater long enough for the motor to warm up so the thermostat opens allowing the water to flow through the complete cooling system flushing out any salt residue. I know a few guys who after fishing saltwater, have a favorite lake or river boat launch where they simply back the boat down (while still on the trailer) and run the motor enough to flush saltwater out.

The method will depend on the make of the motor, the year of manufacture, and the size of the motor. The early (up until the early 1990s on some) motors did not have any provision for this, (either water intake screens on the side of the gearbox, or a flush fitting on the powerhead) therefore requiring it to be submerged in a barrel. Then the Tohatsu made motors after 1987 for Mercury, Nisson, Evinrude, had the water intake behind and under the motor's cavitation plate, necessitating it to be ran in a tank.

Obviously what can be done on these older motors with a small 4 to 10 hp motor may be hard to do with a 35 or 50hp plus motor. A small motor may be able to be ran in a 5 gallon bucket where a 50 hp and larger will require something much larger and a different method of getting it to the water. We will discuss that later.

Some older small motors did not have a gearbox where it had a neutral/forward or reverse. For these motors, the prop was always in gear. Most however, when ran in a small bucket need to be in neutral if possible, to eliminate splashing water out and then starving the water pump. Even a 6 hp motor in a 55 gallon barrel when in gear and revved to maximum will BLOW water up and out the rear top.

When running a motor in a bucket or barrel, it is imperative that the water level is HIGH ENOUGH on the lower unit to to be sure that the water pump is submerged. These water pumps do not suck water, they have to be submerged (have the water as high as, or slightly above the pump). Take notice of the height your motor is in the water on your boat and use that as a guide.

You have to also realize that anytime you run one of these motors where it is not on a boat in the water, and able to rely on the water to provide needed resistance to the prop, that it SHOULD NOT be revved up much over a high idle, as the engine now has no resistance from the prop/water connection and there is a chance it will "Run Away With Itself", thereby revving so fast that you could do major internal damage to the motor.

The NUMBER ONE requirement is to have a WATER FLOW INTO the motor. All water cooled motors will have a water intake on the lower unit. The older ones could have it through a screened tube in the exhaust housing outlet directly behind the propeller. Later motors will have the intake through slots located above the prop but in the main gearbox housing, centered in the main (largest) section, and they will usually be on both sides of this housing. These can utilize the "MUFF", which is a metal yoke holding rubber cups that cover these water inlets. Usually only one cup will have access fitting to connect to your city water hose.

Now, comes some important information, since most city / household well water is usually about 50# pressure, and your outboard motor water pump may only put out less than 10#, you need to DO NOT turn the water valve on all the way. Some say only 1/4 a turn, but that may vary depending on your water source and the make of the source water valve. I usually just slowly turn it on and watch for when water just starts to flow, coming out of the lower unit exhaust. I may even cut it back some, but remember you need enough water to cool your motor and yet not overdo it. AND in flushing a 5hp motor, it will require way less cooling than a 50 hp one does.

Too much high pressure water could possibly damage the motor's water pump impeller, along with giving a false sense of the water pump working, as you will be overriding the impeller and forcing enough water through the system to have the motor peeing.

In the photos below, you can see the damage caused by running a OMC 9.9hp motor out of the water, with no supplemental water for cooling. And the owner thought his water pump was fine because he had it on muffs (with high pressure), and with the motor peeing, but not running quite right !!!!

The older motors with the inlet behind the prop, will need to be ran in a barrel.

With the above said, it is IMPERATIVE TO CHECK these water inlets to be free of debris before you try to start the motor. And as important, if you have it running but the motor is not peeing, before you get excited and tear it apart to replace the water pump, or just the impeller, CHECK these water inlet screens or the outlet Pee hole for obstructions. The earlier screens behind the prop, are considerably more prone to become plugged with weeds than the later motors, simply because they have smaller holes.

Different Methods, Type One ; c One of the earliest and simplest methods to do a test run on a small outboard motor is to fill a bucket with water and put the lower unit in the water. You will need some form of a mounting to keep the motor stationary, here the normal thing is to use a carpenter's sawhorse. Or if on the boat but out of the water, a large bucket will do, but raised up to match the motor. The water height NEEDS TO BE high enough to cover the water pump intake screen/holes, as seen in the photo below. Also in this photo, you will see wooden blocks UNDER the bucket to accomplish this task of raising the bucket.

This motor is old enough and has no transmission so the prop turns all the time the motor is running, thereby possibly needing water to be replenished as it is thrown out. Also you would have to be careful to not allow the prop to hit the sides of the bucket. This plastic bucket may not damage the OBSOLETE prop, but a metal bucket may do so. And finding replacement props for these old motors me be near impossible AND EXPENSIVE even if you could find one.

In the LH photo below, you will notice the 2" X 4" wooden blocks under the bucket to raise the bucket high enough for the water to cover the water intakes.

The RH photo below is basically the same, only using a modified 50 gallon plastic barrel that can be readily moved to the back of a small boat. It is cut out on the front to accommodate the motor while on a small boat. You will also notice the higher rear section on it (full barrel height), which allows the motor to be ran above a idle speed in gear without throwing much water out. These plastic food grade barrels are readily available for a reasonable price, (got this one for $10).

For those who live on a farm, a cattle watering trough can even be used. I had a cousin who would back his boat/jet motor up to his water trough, let the motor down in the water while still on the boat, and ran this to flush out any saltwater. His old Mercury motor/jet was old enough to not have a flushing system installed.

The Ultimate Run in a Barrel, Type One : To make one of these run barrels seen in the photo below, use a 55 gallon steel fuel/oil barrel, cut part of the top out, but leave about 1/3 still attached. Cut back about 5" along the sides of this remaining top. Now bend this 5" section down into the barrel to form a downward lip. This will allow you to rev up the engine for short periods of time even while in gear without blowing the water up and out of the barrel, (and all over yourself) as this downward baffle diverts the prop water back down into the barrel.

This barrel has been used to run motors up to 25hp, but the chore is they are heavy enough that this is usually a 2 person job. However as I get older and find that I have low back issues, lifting motors over 3hp that high is something I try to shy away from, hence the larger movable barrel as seen above.

One thing to check, is that if you leave the barrel set outside from year around with water in it, and you may have trees nearby, you should check for debris that gets inside. In the photo above you will be able to see on the ground beyond the barrel, the top of a galvanized garbage can that is used to keep debris out when not in use.

Also if you run motors a lot, these barrels seem to accumulate a lot of unburned fuel oil mix, so you need to empty and/or clean it occasionally. If the water is not replaced or cleaned, after running a motor, a black oily foamy oil will be floating on top of the water. What I do if using it after letting it sit for a while is grab a roll of paper towels and float a sheet or two on top, (the towel) then pick it off trapping the oil, repeating the process until most of it has been absorbed in the towels and disposed of. If this gunk is not cleaned off, over time it will sink from the surface.

You can pour a small amount of liquid soap, which can help -- HOWEVER do not overdo it as you will get so many small soap bubbles mixed in the water that the motor's water pump impeller cannot do it's job.

Different Methods, Type Two ; OMC small motors from the the late 1960s until the early 1990s (on some) the main water supply to the water pump comes from a screened tube right behind the prop in the exhaust outlet which was carried over from earlier motors. Water is forced into the screened intake to the water pump by the prop thrust.

Some users report that when running these motors in a barrel, (usually a small one) that the prop has to be installed and usually in gear as apparently they need to get more water pushed to the water pump by the prop to get enough to cool when running at over an idle.

There is no stainless steel intake plate on the left side above the cavitation plate like the previous versions. Aftermarket flush kits for this motor are obsolete and rather scarce. There is really no provision currently available for a easy muff system to run this motor out of a tank. But there is hope. I was able, (by tearing mine down) to figure out a way to drill/tap/modify the lower unit near the water pump to where I may be able to utilize the later screw in flush adapters on this motor. But that is a project in itself covered HERE

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 facing backward. A few of these have this plates were painted over originally and little evidence that it had ever been off.

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.

In the LH photo below, 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, being

"Freshwater Flushing Kit" #0435299. Honda also has a Flush Kit, #06190-zv1-860, and all cost around $15.

These adapters also fit about all of the Japanese outboards that use a back-flush system.

In the RH photo below, you see a readers removable unit made from scrap metal and a trip to Home Depot.

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 1/4 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	Here you see a readers Red-Neck removable version

Here you see another readers version

Different Methods, Type Three ; .OMC small motors from the the late 1960s until the early 1990s (on some) the main water supply to the water pump comes from a screened tube right behind the prop in the exhaust outlet which was carried over from earlier motors. Water is forced into the screened intake to the water pump by the prop thrust.

Aftermarket Water Flushing Adapter Cover : This method of water intake does not allow the newer type of flushing muffs to be used. And soon after these motors came out with this type a water intake system, Tempo made Flush Kits #918FAA (as seen in the photos below) for a while for these motors, but now are not currently made and are rather scarce IF THE PERSON EVEN RECOGNIZES WHAT THE ARE. Those of you who frequent garage sales may be on the lookout for these.

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.

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

Different Methods, Type Four ; OMCs 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 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".

AND SOME small Japanese motors are NOT DESIGNED TO ACTUALLY RUN the motor when using them, but to ONLY back-flush from a NON-RUNNING motor, which is better than not at all. AGAIN Some motors, especially the Tohatso imports (Nisson, Mercury and Evinrude) you CANNOT run the motor when using these flush adapters. Read your owners manual.

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".

Different brands have different ideas and locations. In the photo below, top center is a well marked FLUSH plug, the plug on the right is the OIL FILL plug. Both are well marked.

Different Methods, Type Five ; The method most used on later motors is using a "MUFF". These are commonly used to flush a motor after it has been ran in saltwater, or as a test run.

I personally like to start my motor and run it the night before I take it out on a fishing trip, just to be sure it starts for one, and to also be sure that I will not have any problems when it hits the water. This simple test run has saved my butt numerous times. These units can handle motors upwards into the 200 hp plus size.

   These are simply a device that straddles the lower unit putting enough tension on what appears to be large rubber suction cups that cover the water intakes. You attach a garden hose to your faucet and the other to the muffs, TURN ON THE WATER and start the motor. Only a few seconds of running a motor WITHOUT water can AND USUALLY WILL ruin the water pump impeller.

   These are normally used while the motor is still attached to the boat on the trailer.

   The secret here is to not run full force water into the muff, as you can blow water into areas that are not normally seen by water. These outboard motor water pumps do not supply water in a lot of pressure, so cut back on the amount you allow in.

Again it is not a good idea to run any motor at a high speed for more than a few seconds if not on the boat and in the water.

On the motor in the photo above you will see a Prop Guard AND a Happy Troller plate, along with a Jack Plate used to raise the motor to be able to run in SKINNY water.

Here you see what is left of an impeller, ran with not water	Here is the bottom view of the same water pump

Here using an old (1947) small 2.5hp motor & a 5 gallon bucket of water to test run	Here you see a larger version with a 15hp Johnson in it
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Here is the flush plug on a 1994 Johnson 9.9hp	Shown here is the flush hose on a Yamaha T8