How to do an engine compression test

What is engine compression testing

When you notice that your bike engine is running rough or there is a noticeable lack of power or performance and after you can’t find any problems with the fuel or exhaust system, ventilation (breather hoses), or the ignition circuit (wiring, spark plugs, ECU, ignition coils etc…). Then it’s likely that the poor performance could be worn piston rings, worn valves, blown head gasket or cracked cylinder head. It’s also worth doing a compression test before and after you make any modifications such as changing the camshaft.

A compression test measures the amount of pressure generated by the pistons in each cylinder, the combustion chamber must be airtight in order to achieve optimum pressure for detonation. Without this pressure there wouldn’t be enough force to turn the crankshaft.

To do this you will need a compression tester/ gauge to carry out this check.

Compression testing an engine

A compression test can be done when the engine is either warmed up or cold, a hot engine is better for accurate results as all the materials in the engine will have expanded under the heat to form better seals. But if you want to minimise the risk of burning yourself while working on the engine, you can do the cylinder compression tests while the engine is cold – if you choose to do this then remember to add 5% to your readings to allow for the heat expansion effect on the seals in the cylinder and engine.

remove the petrol tank

disconnect the ignition coils

connect the compression gauge

check compression values in the service manual

  1. First you must disconnect the ignition system to prevent electrical burnout of the components as when you disconnect and remove a spark plug, the voltage generated in the ignition coils has no way of getting released and baring in mind that this will be around 30,000 volts then you don’t want this building up in your iginition circuit as it could also cause damage to the ECU. Quickest way to ensure this doesn’t happen is to disconnect the iginition coils.
  2. Next remove the spark plug from the cylinder that you want to test.
  3. Screw in the compression tester in to the cylinder – always worth having  adapters/convertors for your compression gauge to allow for different sized spark plugs etc…
  4. Open the throttle fully, otherwise your figures will be out
  5. Hit the start button or use the kickstart to turn the engine over for around 5 seconds or until the needle on the engine compression tester is at it’s maximum reading.
  6. Take the reading and remember to add 5% if you did this with the engine cold.
  7. Refer to your service manual for the recommended compression results
  8. Typical values will be around 110 – 190 psi depending upon the engine size – generally the higher the compression your cylinder produces then the better (although too high of course will lead to blowing gaskets or cracking cylinders).
  9. Compare the readings from all cylinders and ensure that there is no more than a 10% deviation between cylinders and if there is investigate the cylinder with the lowest compression results first.

How do you check if you have no service manual?

It does happen, either your manual doesn’t have the figures or you have no service manual to hand. As long as you know what the compression ratio is for a cylinder then you can work out the theoretical pressure it should generate as follows:

Working in PSi, multiply the compression ratio by 19 and to measure in Bar then multiply by 1.38 instead – although these figures are very rough and are no substitute for a service manual.

Low readings from the compression check?

If the compression reading is significantly lower then this will indicate an issue with the following:

  • Faulty head gasket
  • Valve seats are worn and the valves no longer fit properly
  • Piston rings are worn/ broken
  • Valve clearances could be out

Once you have all the readings from the compression test if only one cylinder is low then this usually indicates a bad valve, more commonly the exhaust valve or the piston rings are worn. If you get low readings on to adjacent cylinders then this could mean the head gasket will need replacing.

Leak down testing

You can test all of the above using a leak down tester which essentially works by pmuping compressed air into the cylinder chamber and seeing how much of the air is lost – most service manuals will tell you how much leakage is acceptable e.g. 5 psi per 2 minutes.

To do the leak down test the piston must be at TDC (Top Dead Centre) and the settings for how much air at a certain pressure can be found from the manufacturer of the leak down tester.

To figure out what the issue is you can listen out for the escaping air or by using something really light like rice paper that will move in even the slightest breeze.

  • If the air / sound is coming from the exaust, this will be the exhaust valves
  • If it’s coming from the air filters, this will be the inlet valves
  • If you remove the seal from the crank case breather and the oil filler cap and hear the air escaping here then this will be the piston rings

Other ways to check faulty compression

If you want to quickly check the piston rings, or don’t have access to a leakdown tester, then after the first compression test on the cylinder add a small amount of engine oil via the spark plug hole before performing a second engine compression check so this will then create a better seal around the piston and if you then end up with higher readings you’ll know that it’s the piston rings. If the readings aren’t higher then you know that it’s  probably the valves.

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