Runaway Outboards When Using Muffs

This may be my shortest article, but may be valuable if you encounter it happening to you or to someone near you.  When using muffs to test/run/flush a 2 cycle outboard, you may encounter a
"Runaway Engine" at times.  This will be a GREAT SURPRISE to most outboarders.

First off, DO NOT run then over a high idle (1500RPM) while on muffs.  The reason is, the motor is not operating as it was designed like being on a boat and in the water.  In use when hung on the transom, the motor's lower unit and exhaust housing is sitting in water which puts a designed backpressure on the exhaust system.  When using muffs, this backpressure is not there, and if other things are just right/wrong on a carboned up motor, it could runaway by itself. 


I have never had it happen to me, but others have said the motor will run at the top of it's capability for as long as it has fuel and there appears nothing you can do to stop it.  You can disconnect the fuel line, pull the spark plug wires and it just keeps screaming for what seems like an eternity.  If you do not get it shut down SOON, it could damage or even blow up the engine, usually by sending a broken rod right out the side of the block.


Basically you are having pre-ignition within the cylinders.  This can usually be traced to excess carbon buildup on the head or top of the piston that is smoldering from the previous firing.  These chunks of carbon, becoming hot from any running, will ignite the fuel just like a Glo Plug does on model airplanes, igniting the fuel before the spark plug does.  As long as it has fuel, it will run, and usually extremely fast.

Disconnecting the fuel line does nothing immediately because of any fuel inside the carburetor bowl.


About the only way to stop it is for you to grab a shop towel and stuff it very tightly over/into the carburetor silencer/breather intakes, restricting or stopping any air from being sucked in.

This can be extremely interesting, especially if you happen to be working on a friend's motor, or worse if it happens to be a customer's motor.

Actual Experience From a Marine Mechanic; 
"The reason I am responding to you is that I had an outboard Mercury run away while on a flush device about 10 years ago and it really got my attention.  I pulled all the spark plug wires off and she kept on running, and I got lit up in the process.  I have no idea how high it turned.  I figured it would explode.  I unplugged the fuel and it finally cut off.   Throttles closed, wires off, whew!  I just knew I was going to see it's rods any second and that the motor would be damaged.  It was really turning up.  Not so.   WOW!  You have to appreciate my amazement and horror when this motor continued to run with no spark.  We work so hard to provide fuel and spark to these motors and now discover one that can run without spark.  Hum....


I suppose it is still in service.  I couldn't understand how the combustion chamber got hot enough at idle speed and a bit over idle to do that.  I figured I dodged a bullet on that one.  You said you had not seen that in person.  You don't want to either, especially if it is a customer's motor. 


Thanks for the articles.  I'll read more as time permits. Tom Harris, Harris Marine, Cordele, GA., (39 years of Mercruiser service, about 20 of Mercury)"

Decarb ;  Now to prevent this from happening again, either tear it apart and clean the complete combustion chambers, OR the easiest is to decarb the engine, which dissolves this carbon, then burns it.  For this instance, we are trying to remove any internal carbon deposits in the combustion chambers.  Part of this could be from the rings not sealing completely which allow excess oil to bypass, contributing to unburned oil in the combustion chamber, which could then become carbon buildup. 


This carbon buildup can hamper the running of the motor because of lower compression The easiest way to help here would be to purchase a commercial de-carbonizing liquid that is ran thru the gas and / or squirted into the pistons, let set, then run it.  One that is usually available at automotive parts stores is SeaFoam.  This stuff softens the carbon buildup and when the motor is then ran it burns and blows this now softened carbon out the exhaust and many times gives the motor a new lease on life.  

One method of decarbing used by many is to mix a strong mixture of SeaFoam (like 6 oz.) into a 1 pint of mixed fuel, make up a small fuel tank with fitting to the motor if applicable and run the motor at a medium speed, let most of the fuel be consumed and shut the motor off.  Pull the spark plugs, squirt more SeaFoam directly into the spark plug holes.  Let the motor set for 20 minutes, then start it up.  If you have carbon internally the motor will burn this loosened carbon which will SMOKE considerably.  Don't do it in your residential driveway as your neighbors may call the fire department.  And your concrete driveway will be stained black.


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Originally stated 04-19-2013, Last Updated 12-21-2014 
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