Feeling the pressure…
Check the health of your engine with this essential test
1 Check the values
Checking the compression on an engine is an essential part of fault diagnosis. You are measuring the efficiency of the piston and rings, and bore and valves as they compress the air fuel mixture. Worn rings or a damaged piston will give low readings. Check the workshop manual for the specific procedure and the correct compression values for your bike.
3 Strip to access plugs
Many bikes will require the tank and airbox to be removed in order for you to access the spark plugs and there will be a specific procedure that involves un-clipping fuel and breather pipes, and sensors. Make sure you follow the specific procedure for your bike. This 2014 Honda CBR650F only requires the middle fairing to be removed to have access to the plugs.
2 Get up to temperature
Once you have checked the manual, warm the bike up to the normal operating temperature. This is necessary because the piston and rings, along with the wall of the cylinder, expand with the heat and give the correct clearances. Once it is up to temperature, turn the bike off and begin to remove the parts needed to gain access to the spark plugs.
4 Pop your caps
Some bikes will be fitted with plug cap coils and others the traditional plug cap. Remove each one from the spark plug and make a note of where each cap went; often there are numbered tags on the HT leads that relate to each cylinder, typically a four-cylinder bike will number the cylinders 1,2,3 and 4 from left to right as you would view it sitting on the bike.
5 Pull the plug
Your workshop manual will also, in most cases, specify that the compression test is done with all of the other plugs removed. However, the test procedure in our Honda manual instructs you to do one cylinder at a time with one plug out for that cylinder. Remove the plug/s with a dedicated plug adapter socket.
7 Screw in the adaptor
When the plug/s have been removed, you need to attach the compression tester by screwing it into one of the spark plug holes. Most test tools come with a variety of adaptors that fit a specific size of spark plug hole. Reference against the removed spark plug to see that you have the correct adaptor fitted.
9 Note the values
Write down the value from the gauge and repeat the test on the remaining cylinders. Make sure you have everything written down for each specific cylinder. Compare the values with the specified figures in the workshop manual. Any figure that is too low or to high will need further investigation.
6 Make it safe
Check to see if there is a requirement to disable the fuel pump/system. Additionally, there may also be a need to disable the high-tension current that feeds the spark plug caps. Both of these can be done safely by removing the corresponding fuses, check carefully which fuse/s are the relevant ones to pull out.
8 Turn over and take a reading
To give a reading the engine needs to be turned over. This is the part of the test that needs to be checked in the workshop manual, as the procedure can change from bike to bike. When you are ready, turn the engine over for a short period of time with the throttle wide open and wait for the reading on the gauge to stabilise.
10 Rebuild and fire up
When the test is complete re-fit the spark plugs and torque them up to the correct spec. Replace the spark plug caps/coils, making sure that they are the correct number for each cylinder. Fit the airbox and petrol tank then start the bike to make sure it runs. Finish off by re-fitting any panels.