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 using a Motion Pro 4-stroke leak down tester.
Tools Needed – Motorcycle Engine Leakdown Test
- Spark plug wrench
- Leak down tester
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.
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.
- 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.