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Engine compression check

by Had Robinson

Note: Engines with flash starters cannot have their compression easily checked.  See the notes below.

If you want trouble-free operation, you must keep track of the compression of your engine on a regular basis.  It's like going to the doctor.  Regular checkups can save your life – including keeping your paramotor purring for 100's of hours.

Checking the compression will give you the best evaluation there is of your engine's overall mechanical condition.  If it is below the range given in the specifications for that engine, a top end rebuild is necessary.

You must use a good quality compression tester that screws into the cylinder head.  Note that if you use a different tester each time you check your compression, you will get different values so always use the same one.  Carefully follow the directions that came with your tester.  The values you get will be different for a cold or a warmed up engine.

If you have an engine with a flash starter, you must have a heavy duty electric drill with a 1/2" chuck, the correct sized socket, and a socket adapter.  Remove the starter and use the drill to turn the crankshaft at 300 RPM.  There is no other way to check the compression of an engine with a flash starter.

The compression should be about 130 psi (4,000' MSL) or 150 psi (sea level) for engines with a 10:1 compression ratio, like the Top 80.  For other engines go to their respective specification page for the compression ratio and multiply this by 15 to give the approximate compression value.  The exact value will vary depending on altitude, type of compression tester, or whether it is a cold or hot engine.

Engines that have a decompression port (but not a flash starter) like the Minari and Simonini will have much lower values (about 10-20% less).

FLASH STARTERS  Unfortunately, compression cannot be checked on engines that have Flash starters because the flash mechanism will not turn the engine over fast enough to measure the compression accurately or consistently.  This is by design with the flash mechanism.  It allows the compressed fuel mixture to leak off enough to slowly (and easily) get the piston to almost top-dead-center (TDC).  The flash spring then quickly turns the crankshaft past TDC fast enough to generate an ignition spark, which ignites the fuel mixture and starts the engine.

If the compression is less than 90% of the correct value, replace the cylinder, piston, and ring.  Miniplane-USA sells kits that includes these parts.  Pilots can also have their cylinders re-bored at this shop for a modest price and get an oversized piston from Miniplane-USA.  Cost-wise, the first option is more economical.

A compression check, however, will not tell you what condition your engine bearings are in.

Remember that idling a paramotor for more than a few minutes at a time will wear out clutch parts prematurely and foul your spark plug.

If you fly in a dusty place or like doing foot-drags in sandy locations (e.g. the beach), expect dramatically increased wear in your engine top end.  It is almost impossible to keep sand out of an engine if you launch from a beach.  Always launch from a wet, grassy area, if possible, or from a paved area free of sand.