The Buckshot Racing #77 Compression Tester Gauge checks cylinder compression on all outboard and race engines up to 300 psi. It's equipped with an air-hold valve and an easy gauge reset button. The 10-inch flexible rubber hose reaches each cylinder and fits all outboard spark plug 14mm threads.
 
More flexible to use and less money compared to the Mercury Marine Quicksilver 91-29287 Compression Gauge with an MSRP of $91.99.
 
Instructions:
 
To check the compression on a 2-stroke outboard, you can try these steps:
 
Remove all spark plugs, clean, and dry the spark plug
Screw in the compression tester finger tight, one cylinder at a time
Ignition off, open the throttle to 100%
Crank the engine about 4x, until the gauge stops increasing
Record the reading
Repeat for each cylinder
 
Here are some tips for checking compression on an outboard engine:
Warm the engine before doing the test
Count the pulses when cranking
Use the same amount of pulses for each cylinder
 
A good compression reading for a 2-stroke engine is between 110 and 130 psi, as the color change from red to green indicates on the Buckshot Racing #77 Compression Gauge.
 
The difference between the highest and lowest readings should be no more than 10%. Low compression can be caused by a leaking piston, piston ring, camshaft, head gasket, or valves. To test for this, you can add some oil to the cylinder.
 
If compression is low, it's recommended not to run the engine. Pull the cylinder for further inspection. It could be time for a build before more damage is done by running the motor.