What Causes an Outboard Motor to Flood?

Written by william brown
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What Causes an Outboard Motor to Flood?
Outboard motors power small sport and pleasure boats such as this pontoon boat. (Justin Sullivan/Getty Images News/Getty Images)

Outboard motor flooding is caused by an imbalance in the three items needed for combustion: fuel, air and heat. The heat is provided by the spark plug and the compression of the fuel-air mixture. The fuel and air are mixed in the carburettor before going into the cylinder for combustion. Flooded outboard motors have too much gasoline in the cylinder to ignite properly. Flooding is alleviated by correcting the imbalance of these three items.

Improper Use of Choke

A manual choke is a metal valve in the throat of the carburettor. During regular use, the choke is parallel to the sides of the carburettor and does not impede the carburettor air supply. When the choke is engaged, it turns 90 degrees to become perpendicular to the throat. This cuts down on the amount of air being mixed with the fuel. If the choke is not disengaged promptly when the engine starts, flooding occurs.

Improper Use of Priming Pump

Outboard motors have a priming pump installed in the fuel line to the motor. It is a bulb-shaped device that is squeezed by hand to pump fuel into the outboard motor before it is started. When done correctly, the fuel in the cylinders will ignite and start the engine. If the priming pump is pumped too much, the excess fuel will cause "wet spark plugs" and foul the spark plug so a clean ignition is not possible.

Ignition System Needs Tuneup

A dirty or untuned ignition system leads to flooding. Pull out the spark plug with a spark plug socket that has a rubber insert to prevent cracking of the plug's ceramic insulation. Clean the spark plug. Check the spark plug gap and reset it to the gap specified in the owner's manual if needed. Check the ignition points for carbon build-up or pitting of the points. Clean or replace the points as needed. Set the ignition gap per the owner's manual.

Low Cylinder Compression

Compression in the cylinder raises the air-fuel mixture temperature to help the spark plug ignite the mixture. If it is too low, the engine can start but the incomplete combustion of the fuel will build up on the spark plug and cause the engine to sputter to a stop. Check the cylinder compression with a compression gauge. If it is below the specifications in the owner's manual, a rebuild is needed. Dissemble the engine to remove the cylinder and install new rings. Replace the cylinder head gasket.

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