Engine compression check

by Had Robinson
updated August 10, 2023

Note: Engines with flash starters require additional steps to check the compression.

If you want trouble-free operation, keep track of engine compression 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.

The important thing about an engine's compression is what the value is when it is new, whatever it is, and what it is after it has been run, let's say, for 50 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, at the minimum, is necessary.

Use a good quality compression tester that screws into the cylinder head.  Note that use of a different tester will give different values.  Carefully follow the directions that came with the tester.  The values are different for a cold and hot engine.  Fully open the throttle when performing the test.  If this is not done, the reading will be low.  Most Italian engines have a 14mm spark plug so the tester must have a 14mm adapter.  Only the Top 80 has a 10mm spark plug and it can be difficult finding a tester with a 10mm adapter.

If there is any sort of washer or ring that is normally installed with the spark plug, remove it.  These washers/rings are used to lower the compression to the correct value.

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.

The compression should be about 130 psi (4,000' MSL) or 150 psi (sea level) for engines with a 10:1 compression ratio, like a Top 80.  For other engines go to their respective specification page (or the engine manual) 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 easily 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 air 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.  To measure the compression of engines with flash starters, the starter must be removed and the engine rotated using a powerful handheld electric drill, an adapter with the correct sized socket to fit the flywheel nut.

If the compression is less than 90% of the correct value, replace the cylinder, piston, and ring.

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 the 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.  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.