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 120 and 140 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.
 
🔧 Quick Compression Test Summary
 
0-90 PSI – Rebuild required: likely worn rings or scored bore.
90–100 PSI – Borderline: engine may run but is tired, expect rebuild soon.
100–120 PSI – OK for recreational use: typical for older stock motors.
120–140 PSI – Strong engine: good sealing, run premium (91+) octane fuel.
145-155 PSI – High-performance build: must run premium (93+) or race fuel.
155-210 PSI – Race motor territory: use race fuels or blends (96+) octane.
 
✅ Always check that all cylinders are within 10 PSI of each other.
 
⚠️ Mismatched readings or a single low hole may indicate a ring, port, or piston issue.
 
⛽ Match fuel octane to compression—high PSI needs high-octane to avoid detonation.
 
⏱️ Timing – Keep ignition timing at 25° BTDC or less, unless tuning for a race setup.
 
Note: The timing guidance provided (25 degrees BTDC or less) applies specifically to Mercury 2.0, 2.4, and 2.5-liter 2-stroke engines. For other motor models, always refer to the factory timing specifications.
 
For technical support, contact Mike Hill at +1-714-697-1716 or mike@buckshotracing77.com