Why it's the first test on any used or sick engine

A compression test takes ten minutes and tells you whether the powerhead is worth diagnosing further. Do it before quoting carb work, tune-ups, or any pre-purchase inspection — no exceptions on two-strokes.

Procedure

1. Warm the engine if possible (cold numbers read lower but are still useful for comparison between cylinders).
2. Disconnect the battery-side risk: remove the lanyard, disable ignition per the service manual (ground the plug leads or disconnect the kill circuit so the packs aren't damaged and nothing fires), and shut off fuel.
3. Remove all spark plugs — note which cylinder each came from and read them (oily, chalky white, wet, aluminum speckles = each tells a story).
4. Thread the compression gauge into cylinder 1, hold the throttle wide open, and crank through several compression strokes until the needle stops climbing.
5. Record, repeat for every cylinder with the same number of cranks.

Reading the numbers

• Evenness matters more than the absolute number. Gauges vary; cylinders on the same engine should be close to each other — a cylinder noticeably lower than its siblings is the finding. Check the service manual for the acceptable variation and any published minimums for your model.
• One low cylinder on a two-stroke: suspect stuck/broken ring or scoring — look through the exhaust ports or borescope through the plug hole before condemning.
• Two adjacent low cylinders on a four-stroke: possible head gasket.
• Zero on one cylinder: hole in a piston, dropped valve, or a stuck-open thermostat of the mechanical kind — borescope it.
• A wet test (small squirt of oil, retest) that raises the number points at rings; no change points at valves (four-strokes) or head gasket.

Common mistakes

• Testing with throttle closed (starves airflow, lowers all readings)
• Different crank counts per cylinder
• Condemning an engine on absolute PSI from an uncalibrated gauge
• Forgetting that a low battery cranks slow and reads low across the board

When to walk away

On a pre-purchase inspection, one dead cylinder ends the inspection — report it, don't open the engine on the seller's dock. On repair calls, a scored two-stroke cylinder means quoting a powerhead job or repower, not a tune-up.

Safety: ignition disabled and fuel off before cranking; plug wells can spit fuel vapor.