4-Stroke Motorcycle Engine Leak Down Testing

When a motorcycle refuses to fire up, a compression test often becomes the go-to diagnostic. However, sometimes it’s better to do a leak down test instead.

A compression test gives you a snapshot of the engine’s health and tells you if the cylinder is sealing properly, but not much else. On the other hand, a leak down test gives you a more comprehensive diagnosis, including whether the piston rings are leaking or whether the valves are sealing properly. A leakdown test paints a better picture of what’s going on with the engine. Here’s how to do a leak down test on a motorcycle engine.

Tools – Motorcycle Engine Leakdown Test

Buy cylinder leak down tester

Buy spark plug wrench

How to Perform a Leak Down Test

Step 1. Set the engine to top dead center on the compression stroke so all the valves are closed.

Step 2. Remove a spark plug and attach the leak down tester to the cylinder.

Step 3. Connect an air compressor to the leak down tester and set it to a specific pressure.

Motorcycle engine leak down test

How to do a motorcycle engine leak down test

Step 4. Release the leak down tester’s valve so the air flows into the cylinder. The tester will show the pressure differential, as a percentage, between the incoming air and the cylinder.

Motorcycle engine leakdown test how-to

  • 5% or less - No leaks, the engine is in excellent condition and no repairs are necessary.
  • 5% - 15% - Minor leaks, the engine is in average condition and repairs may not be necessary yet, but probably will be soon.
  • 15% or more - Major leaks and the engine is in need of repairs.

Pinpointing Leaks Using a Leak Down Tester

With the tester still keeping the cylinder pressurized, listen or look for where the leak or leaks are coming from.

  • Leaking intake valves - Sound of air escaping at the carburetor or throttle body.
  • Leaking exhaust valves - Sound of air escaping through the exhaust pipe.
  • Leaking piston rings - Sound of air escaping through the crankcase breather hose. (Alternative is to remove the oil fill cap and listen for sound of air escaping into the crankcase).
  • Leaking head gasket - Sight of bubbles escaping from the radiator coolant.

As these diagnoses illustrate, performing a leak down test can quickly give you a detailed picture of the engine’s condition and let you know whether the problem is something you’ll need to open up the engine to rectify. 

If you want to learn how to do an engine compression test as well, watch the video above to see one done on an ATV.



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