Boat › Outboard › Powerhead › Crank but no start › Low compression

In order to run and start the pistons must be able to compress the air drawn into the cylinder sufficiently to get the correct air to fuel ratio needed to start and run. If an engine has extremely low compression you will hear the starter motor spin it very quickly or you will feel little resistance on the pull cord.

A compression test is a good way to check the general health of an engine. In this video I do one on a Yamaha 50HP four stroke. Around 120 PSI is good; if the compression is much lower, or varies a lot between cylinders, there's not much point looking at other problems until that's fixed. Low compression can come from worn piston rings, a blown head gasket or, on a four stroke, valves that aren't seating properly.

A leak-down test on the same motor, to find out where the compression is going. It points to a burnt exhaust valve on cylinder one, and maybe two.

The head gasket on the engine is blown

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There is a gasket that seals the cylinder head to the engine block to prevent the combustion gasses escaping and to keep the cooling water (and oil on a four stroke) out of the cylinders.

Worn or broken piston rings

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Piston compression rings primarily function to seal the cylinder walls, containing the air fuel mixture so it can be compressed as the piston rises. If the compression rings are worn or broken, compression will be low.

There is a hole in the top of the piston

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A hole melted or punched through the piston crown joins the combustion chamber to the crankcase, leaving that cylinder with no compression. I pull the power head, strip it down and check the pistons and bearings.