How to Perform a Two Stroke Leak Down Test

An airtight engine is critical to proper two-stroke performance and reliability. Leaking crankshaft seals, gaskets, intake manifolds, or crankcase joints can introduce additional air into the combustion process. This creates a lean air/fuel mixture that cannot always be corrected by changing carburetor jets.

A lean-running two-stroke engine may experience hard starting, an inconsistent idle, hanging RPM, overheating, detonation, piston damage, or complete engine failure. Performing a two-stroke leak down test is one of the best ways to locate these problems before they cause expensive damage.

The Ken OConnor Racing Leak Down Tester allows you to test the engine under both vacuum and pressure. This is important because some seals may hold pressure but leak under vacuum—or hold vacuum but leak under pressure.

When Should You Perform a Leak Down Test?

A two-stroke engine should be tested whenever an air leak is suspected. We also recommend testing:

  • After rebuilding an engine
  • After replacing crankshaft seals
  • After installing a cylinder or intake manifold
  • Before attempting to correct a persistent jetting problem
  • When an engine has a hanging or inconsistent idle
  • When an engine runs unusually hot
  • After a piston seizure or suspected lean condition

A leak down test should always be performed before a freshly rebuilt two-stroke engine is started. Finding a leak before running the engine can prevent serious damage.

Preparing the Ken OConnor Racing Leak Down Tester

Thread the supplied hose barb into the threaded hole in the exhaust block-off plate. Apply Teflon tape or a small amount of thread sealer to the threads.

Do not overtighten the hose barb. Tighten it only enough to create an airtight seal. Excessive force can damage the threads or block-off plate.

Before testing the engine, inspect the tester, gauge, hoses, fittings, and block-off components for damage. A leak in the test equipment can produce a false indication that the engine is leaking.

Installing the Exhaust Block-Off Plate

Remove the exhaust pipe from the cylinder.

Install the exhaust block-off plate using the supplied rubber gasket. Apply a small amount of grease to the face of the rubber gasket where it contacts the engine. The grease helps the gasket conform to the exhaust surface and create an airtight seal.

Position the plate evenly over the exhaust outlet and tighten the mounting hardware gradually. Do not overtighten the plate or crush the rubber gasket.

Sealing the Intake

Remove the carburetor as necessary to access the engine intake manifold.

Select the supplied white pipe cap that fits the intake manifold. Insert the cap into the manifold and tighten the original intake hose clamp around it.

Make sure the cap is fully seated and the clamp is tight enough to prevent leakage. Do not overtighten the clamp to the point that it damages the intake manifold.

Install and tighten the engine’s spark plug before continuing.

Connecting the Tester

Connect one of the supplied tubes to the hose barb on the Ken OConnor Racing Leak Down Tester.

Connect the opposite end of the tube to the hose barb installed in the exhaust block-off plate.

Inspect both connections to make sure the tube is completely seated and free from kinks, cracks, or damage.

Two-Stroke Vacuum Test Procedure

Move the black sliding check valve on the front of the tester forward to select the vacuum function.

Continue to operate the pump handle until the gauge reads:

14 inHg of vacuum

Stop pumping and watch the gauge. The vacuum should stabilize and hold for two minutes.

A noticeable loss of vacuum indicates a leak in the engine, block-off components, hoses, gauge, or tester fittings.

Do not continue pumping beyond the specified vacuum level.

Two-Stroke Pressure Test Procedure

Move the black sliding check valve backward to select the pressure function.

Slowly pump the handle until the gauge reads:

7 PSI

Stop pumping and monitor the gauge for two minutes. The pressure should stabilize and hold without a noticeable loss.

Do not exceed 7 PSI. Applying excessive pressure can damage crankshaft seals or force them out of position.

Finding a Two-Stroke Engine Air Leak

If the engine does not hold pressure or vacuum, use the pressure function to locate the leak.

Pressurize the engine to 7 PSI and spray a soapy-water solution around every possible sealing point. Escaping air will create visible bubbles.

Check the following areas carefully:

  • Tester gauge and fittings
  • Tester hose connections
  • Exhaust block-off plate
  • Intake block-off cap
  • Spark plug and spark plug sealing washer
  • Intake manifold,
  • Reed cage and reed cage gasket, THIS IS THE MOST COMMON PLACE TO FIND A LEAK ON A FRESH REBUILD
  • Cylinder base gasket
  • Cylinder head gasket or O-rings
  • Crankcase center seam
  • Magneto-side crankshaft seal
  • Clutch-side crankshaft seal
  • Oil injection fittings or block-off components
  • Any crankcase plugs, fittings, or sealing surfaces

Always check the tester, gauge, hoses, and block-off components first. A leaking test connection can easily be mistaken for an engine leak.

Checking for Internal Engine Leaks

Some leaks may be internal and will not produce bubbles on the outside of the engine.

Watch for bubbles or escaping air at the transmission vent tube during the pressure test. Air coming from the vent can indicate an internal leak, such as a leaking clutch-side crankshaft seal or a problem between the crankcase and transmission cavity.

A magneto-side crankshaft seal leak can usually be found by spraying soapy water around the seal behind the flywheel.

Understanding the Test Results

An engine that holds both 14 inHg of vacuum and 7 PSI of pressure for two minutes has passed the basic leak down test.

If the gauge reading drops, locate and repair the leak before starting or operating the engine. Do not attempt to compensate for an air leak by installing larger carburetor jets. Jetting changes cannot properly correct unmetered air entering through a failed seal or gasket.

After completing a repair, repeat both the vacuum and pressure tests. The engine should pass both tests before it is considered ready to run.

Why Pressure and Vacuum Testing Matter

A running two-stroke engine continually produces alternating pressure and vacuum inside the crankcase. Testing in only one direction may fail to reveal a damaged seal.

The Ken OConnor Racing Leak Down Tester checks the engine under both conditions, providing a more complete evaluation of crankshaft seals, gaskets, intake components, and crankcase sealing surfaces.

Testing the engine before operation is inexpensive protection against lean running, overheating, piston damage, and premature engine failure.

Get the Ken OConnor Racing Leak Down Tester

SHOP KEN OCONNOR RACING LEAK DOWN TESTER

The Ken OConnor Racing Leak Down Tester includes the components needed to perform pressure and vacuum testing on compatible two-stroke engines.

Use it after an engine rebuild, when diagnosing a suspected air leak, or before making carburetor jetting changes. Confirming that the engine is airtight gives you a reliable foundation for proper tuning and dependable performance.

Protect your engine before you run it—test it first with the Ken OConnor Racing Leak Down Tester.