On an outboard two stroke engine, what tests can I do with a compression tester, (and how to carry them out)? It's a Yamaha 200 hp V6 2-stroke boat engine.
As is said here 2-Stroke Squish & Compression Testing
The Best Way to get a Valid Compression Reading
A) Within 5 days of a normal “operating cycle - Engines that have sat for a long time may have excess oil that has leaked in from somewhere. The added ring sealing offered by this excessive oil presence can greatly increase the normal valid reading.
B) Cold Engine only - Hot engines will yield lower numbers, and varying levels of temperature will yield varying levels of lowered readings.
C) Fuel Petcock “off” - To avert fire risks.
D) Throttle “must” be full open - To consistently allow the same air access to the lower end.
E) Spark plug mounted in cap and grounded to motor (to avoid harming ignition)
F) Put your machine in 1st or 2nd gear (use the same gear each time), and push the bike at a walking speed for about 10 feet (road-racers with very tall gearing should go 20ft). Doing the test in this way assures that the engine will always spin at the same speed, and that gauge gets enough "pressure hits" from the cylinder to correct for a gauge that may have an unusually large volume hose.
G) Repeat - It’s not a bad idea to do two or three tests (back to back) to assure consistency
I have tested 2 engines following this guide, but never marine ones