When your chasing an engine performance problem, running a compression test early can answer a lot of questions. I’m a mechanic for over twenty years and I can’t run my shop without a compression test kit and a leak-down tester.
ATV engine compression specifications vary by model. They range from 100 – 240 psi (Pounds per Square Inch).
In this post you’ll learn the importance of compression, how to test it using two different methods and what the different results actually mean.
You already know how important compression is, and if it’s not to spec it can cause all sorts of problems. Common symptoms include:
- No starts
- Rough running
- Blue/white smoke trail
- Misfiring engine
- Plug fouling
- Constant oil top ups
- Oil leaking
- Blown head gaskets
Compression as you know varies from make and engine size and the values will decrease with mileage.
Checking compression is pretty simple but there’s a procedure we’ll need to follow to get it right. After all you’ll be basing some major decisions on the outcome of this test.
It is worth noting however, some engines have a cylinder decompression assembly that may interfere with readings.
What’s a decompression assembly? It’s an assembly inside the engine that opens the valves just a little earlier than normal, when cranking over the motor.
It’s function is to reduce compression at start up. This reduces fatigue on the battery and starter motor and helps the engine start quickly.
On these type engines a leak down tester may be a better tool, and I’ll cover that below (Check your manual and see what type engine you have).
Anyway, no matter which engine you have, I’ll show you how to check for major engine issues using just a compression tester.
Dry Compression Test
This is known as a dry compression test and when we get to the wet test you’ll understand why.
Begin with a fully charged battery, a weak battery will cause poor readings. Place the bike in neutral with parking brake on.
A test kit is required and it consists of a gauge with pressure release valve, and a quick coupling plug hole adaptor. A kit will usually include the most common size plug adaptors.
1 To check compression, it’s best to start with a warm engine. So if your bike runs, allow it idle for a few minutes.
2 Remove the spark plug wire and spark plug. Refit the plug wire to the plug and ground it (Remove & ground both plugs if twin cyl). Cranking the engine without grounding the plug wire can cause damaging voltage spikes in the ignition system. (I use a jump cable to securely supply a solid ground to the plug)
3 Select the correct adaptor and screw into the plug socket, just tighten by hand, no tools needed.
4 Connect the gauge using the coupler.
5 Set throttle to wide open and crank over the engine at least six
revolutions. Note the readings, zero the gauge and repeat the test to verify initial reading.
Dry Test is complete, unless your engine is a twin cylinder. If so, repeat test to the second cylinder. A 10% difference between cylinder readings indicates a problem with the lower valued cylinder. Further diagnoses will be required.
Wet Compression Test
A wet test is a useful twist on a standard compression test. I was thought to use this test after first identifying a low compression issue. The dry test results, compared against your engines factory specs will confirm this.
The wet test will help isolate (top or bottom) the cause of low compression.
How to perform the wet test? The wet test is identical to the dry test, with one important exception. Before fitting the test adaptor, pour a cap full of fresh engine oil into the cylinder, you may need to use a funnel.
The purpose of the oil in this test, is to help seal the oil rings.
How to make sense of the results? Very simply, your test will reveal one of two results. (1) Either the wet and dry compression values are very different, or (2) The values are pretty much the same.
(1) A difference of more than (10%) between wet and dry indicates worn rings and you’ll need to rebuild.
(2) Readings that are relatively unchanged mean the rings are not at issue. And a compression issue is likely caused by a top end fault. Areas to check include:
- Tight valves
- Blown head-gasket
- Damaged valve seat or valve
Alternative to a compression test is the leak-down test, it has some advantages over the compression test.
The leak down test is also a really great test to perform, but you’ll need a couple bits of kit. You’ll need a leak-down tester kit obviously, but you’ll also need a compressor.
What’s a leak down test? This test simply involves using a couple of pressure gauges and compressor to pump air into the engine cylinder. The health of the engine is determined by the pressure held over a given time.
How to set up the test? Remove the spark plug, and set the engine to TDC (Valves are closed), this ensures the cylinder is sealed.
Fit your spark plug adaptor and compress the cylinder using your pressure gauge regulator. The twin dial gauge consists of a gauge to measure air pressure in and the other measures the pressure held by the cylinder.
While the gauge is useful and easy to read, it’s often your ears that do the actual detecting.
If the engine is worn the compressed air can be heard leaking, you just have to find it, and interpret it.
Air leaking from the:
- Dipstick – Rings need attention
- Carburettor – Inlet valve/seat needs attention
- Exhaust muffler – Exhaust valve/seat needs attention
- Cylinder head leak – Cylinder head gasket needs attention