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Honda CX500, GL500, CX650, GL650: How to check or test your engine compression

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To do this, you will need some tools to remove your spark plugs and a compression tester (don't have one? eBay, eBay AU, eBay UK, eBay CA and Amazon have some inexpensive options):

Some compression testers won't come with the right connection to screw into a CX500's spark plug holes, so you might need to use a rubber-tipped attachment instead:

Rather than screwing in where the spark plug goes, this type of attachment is held in by the user (you) firmly. A screw-in gauge reduces the chance of an innacurate reading by ensuring a good seal, though depending on where you are they may be hard to find. Locally Supercheap Auto seem to stock a Stanley press-in gauge for around $15. The other two areas of interest on this tool are the release valve:

This tester will retain the maximum pressure reached until you push this little button in, where it will release and return the gauge to zero. Don't forget to do that after you're done checking each cylinder. Speaking of the gauge;

This particular gauge shows kg/cm, PSI and Bar. Most engines we have seen have compression figures expressed in PSI, and the CX500's factory compression is 171 PSI, + or - 10%. Warm or cold engine? A cold engine will give you a worst-case reading as the rings haven't warmed up to their operating temperature (and thus expanded to reduce the gap), though a warm engine is going to give you a measurement accurate to what's happening when the engine is running under normal operating conditions. Given how easy it is on a CX500, we tend to do both. Just don't burn yourself when doing it on a hot engine, and be careful taking spark plugs out of a hot engine - if you feel a lot of resistance don't push it as you don't want to ruin the threads. There is also a sometimes-quoted concern that when removing spark plugs from hot aluminium heads the head will warp and deform the spark plug hole - if you are worried about this it's worth researching first to decide whether you put stock in that concern or not.

Step 01. Remove both spark plugs boots:

Step 02. Remove both spark plugs. This is a good time to check your plugs to see what's happening inside your combustion chamber.

Step 03. Turn kill switch to off:

We don't actually want the engine to try and fire while we're doing this.

Step 04: Insert pressure gauge where the spark plug went, pressing down firmly into the hole with the rubber end:

Step 05: Open the choke (this means the choke lever is in the lower position - e.g. allowing the most air to pass, what most people seem to consider "off")

Step 06: Twist the throttle until it is wide open.

Step 07: Crank the engine using the starter switch while watching the gauge; it will jump up each time the cylinder you are testing reaches the compression stroke, and will do so until it reaches a peak value. Usually this takes 4-10 cycles. Count the number of cycles, and then write down your result next to which cylinder it was. Release the pressure with the button on the side.

This particular CX reads at ~167PSI, which is not half bad for a 30+ year old engine! Is your reading unusually low? Did you remember to hold the throttle wide open? With a closed throttle, the same engine gives this reading:

You need to keep the throttle open so that the bike can draw enough air through to compress it; not doing so will produce low readings. If you can hear air whooshing past the tester you haven't got a good seal, and will end up with a low reading - try reseating the seal and pressing in more firmly.

Repeat the process for the other cylinder:

This is ideally what you want to see - compression figures close to what the factory spec is and the two sides being as close as possible. Low readings can indicate worn piston rings or poor valve clearances. Note also that compression test figures are usually figured based on you testing at or close to sea level - if you're significantly above this your numbers will be lower. High readings can indicate a lot of carbon build-up or oil leaking into/being present in the cylinder. The two sides should ideally be no more than ~10% apart. This is known as a dry compression test - we will do another post on wet compression testing and link to it from here once it is up.