When you are chasing an engine performance problem, running a compression test early can answer many questions. I’ve been a mechanic for over twenty years and 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 mean.
You already know how vital compression is, and if it’s not to spec, it can cause all sorts of problems. Common symptoms of low compression 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 the 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 compression 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.
Its function is to reduce compression at start-up. This reduces battery and starter motor fatigue and helps the engine start quickly.
On these types of engines, a leak-down tester may be a better tool, and I’ll cover that below (Check your manual and see what type of 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 the parking brake on.
A test kit is required, and it consists of a gauge with a pressure release valve and a quick coupling plug-hole adaptor. A kit will usually include the most common size plug adaptors.
- To check compression, it’s best to start with a warm engine. So if your bike runs, allow it to idle for a few minutes.
- 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 jumper cable to securely supply a solid ground to the plug)
- Select the correct adaptor and screw it into the plug socket. Just tighten by hand. No tools are needed.
- Connect the gauge using the coupler.
- 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 the initial reading.
The dry test is complete unless your engine is a twin-cylinder. If so, repeat the test on the second cylinder. A 10% difference between cylinder readings indicates a problem with the lower-valued cylinder. Further diagnosis will be required.
Wet Compression Test
A wet test is a useful addition to the standard dry compression test. I use this test after first identifying a low compression issue. (The dry test results, compared against your engine’s factory specs, will confirm this)
Performing the wet test, therefore, is only required if you have found your engine is low on compression and you are attempting to isolate the cause. ie is the bottom end (piston, rings) or the top end (valves) at fault?
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 do you make sense of the results? Very simply, your test will reveal one of two results.
(1) The wet and dry test values are pretty much similar, with no real difference. 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 to include:
- Tight valves
- Blown head gasket
- Damaged valve seat or valve
(2) The wet and dry compression values are very different. A difference of more than (10%) between wet and dry compression readings indicates worn rings, and you’ll need to rebuild.
An alternative to a compression test is the leak-down test. It has some advantages over the compression test and that’s what we’ll cover next.
The leak-down test is a great test to perform, but you’ll need a couple of 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 involves using a couple of pressure gauges and a compressor to pump air into the engine cylinder. The health of the engine is determined by the pressure held over a given amount of time.
How to set up the test? Remove the spark plug, and set the engine to TDC (Valve 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. The goal here is to find the source of the leak and interpret it.
Air leaking from the:
- Dipstick – Rings need attention
- Carburetor – Inlet valve/seat needs attention
- Exhaust muffler – Exhaust valve/seat needs attention
- Cylinder head leak – Cylinder head gasket needs attention
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