How to Compression Test Small Engines and Two-Stroke Motors

What you need before you start

A compression test measures how much pressure each cylinder builds as the piston rises. Weak pressure points to worn rings, a scored cylinder, a burnt valve on a four-stroke, or a leaking head gasket. It is one of the quickest ways to decide whether a hard-starting engine needs a carburetor cleaning or a real repair.

For gasoline small engines and two-stroke motors, a simple gas gauge is enough. You do not need a diesel gauge. Our best compression tester page lists gas-only kits and larger gas and diesel kits, so you can match the tool to the engines you own.

• A compression gauge with a hose and a pressure release valve
• A thread-in adapter that fits your spark plug hole (many small engines use a different thread than cars)
• A spark plug socket and a clean rag
• Compressed air or a small brush to clear dirt from around the plug
• The service manual or a reliable spec for your engine
• Safety glasses and gloves

Choosing a gauge and adapter

Small engine plug holes vary, so check the thread size before you buy. The BETOOLL HW0130 on our comparison page, for example, is a gas-only kit with a 3 in gauge that reads 0 to 300 psi and 0 to 20 bar. It includes four adapters in 10mm, 12mm, 14mm and 18mm threads, a 17.7 in hose and a quick release valve.

That kit also includes two rubber cone tips that press into the plug hole instead of threading in. They are quick, but a threaded adapter gives a more reliable seal and keeps your hands clear of the hole. On a high-compression engine, the pressure can push a hand-held cone out.

Larger 46-piece kits, such as the DUOYI (700 psi, 18 in hose) and the Wzyblixo (725 psi, 18 in hose), add diesel adapters and a leak-down tester. That is more than a mower or chainsaw needs, but it helps if you also work on diesels or want to find where pressure is escaping.

Safety first

Gasoline vapor and sparks are a bad mix. Work outdoors or in a well-ventilated space, away from open flames and spilled fuel. Wipe up any fuel before you start cranking.

Pulling the plug wire is not enough on its own. Disconnect the wire and ground it away from the plug hole, or switch off the ignition circuit, so the engine cannot fire while you crank it. Keep your face and hands clear of the plug hole, since a loose adapter can eject under pressure.

Let a hot engine cool enough to touch before removing the plug, because the aluminum threads strip easily when hot. Wear eye protection throughout.

Step-by-step: testing a lawn mower or other four-stroke

Run the engine for a few minutes first if you can, then shut it off. A warm engine gives a more realistic reading, but let it cool a bit before you handle the plug.

• Disconnect the spark plug wire and ground it, or use the kill switch method in your manual.
• Clean around the spark plug so dirt does not fall into the cylinder.
• Remove the plug with the right socket and note its color and condition.
• Thread the adapter into the plug hole by hand, then attach the gauge and hose.
• Open the throttle fully. On a carbureted engine this also lets in plenty of air, and many engines also need the choke off.
• Crank the engine with the pull cord or starter for several full pulls, until the needle stops rising.
• Write down the highest reading, press the release valve, and repeat once to confirm.
• Reinstall the plug and reconnect the wire.

Compression testing a two-stroke

You can compression test a two-stroke, and the steps are the same. The difference is in what the numbers mean. A two-stroke has no valves, so a low reading usually points to worn rings, a scored piston or cylinder, or leaking crankcase seals and gaskets.

Because two-strokes build compression differently, compare your reading with the maker's spec for that exact motor. On engines with more than one cylinder, compare the cylinders with each other. A large gap between them is a warning sign even when every cylinder is close to spec.

A leaking crankcase seal will not always show on a standard compression test, since it affects crankcase pressure rather than cylinder pressure. If compression looks fine but the engine runs lean or will not idle, a leak-down or crankcase pressure test can help.

Outboards, snowmobiles and reading the results

For an outboard, remove all the plugs and keep the motor in a safe setup, such as a water tank or a hose supply, if you plan to run it at all. For a compression test you are only cranking, so a short crank is usually fine, but check your manual. Disconnect the kill switch lanyard and keep clear of the propeller.

For a snowmobile, test with the engine warm if possible and the throttle held wide open. Pull the plugs on every cylinder so the engine turns freely, and test each cylinder in turn. Snowmobile and outboard engines often have more than one cylinder, so the cylinder-to-cylinder comparison is your most useful result.

As a general rule, readings that are close to each other and near spec are healthy. If one cylinder reads clearly lower than the others, squirt a small amount of oil into that cylinder and test again. If the number rises, rings are the likely cause. If it stays low, look at valves, the head gasket or the cylinder wall.

Frequently asked questions

Do I need a different gauge for a two-stroke?

No. A standard gas compression gauge works. Just make sure the adapter fits your plug thread and seals well.

Should the throttle be open during the test?

Yes. Hold it wide open so the engine can draw in air and give a true reading.

How many pulls or cranks are enough?

Crank until the needle stops climbing. Several full pulls or a few seconds on an electric starter is typical.

What if my compression is low?

Try the oil test, then consider a leak-down test to see whether the rings, valves or head gasket are leaking.