Compression check is probably the first thing that comes to mind when coming to check a motorcycle. Compression check is done by removing all the plugs in an engine, and inserting a compression gauge on the cylinder. The starter is then engaged, spinning the engine and the piston is producing a compression which is tested.
This is a standard check but it doesn’t really give you a clear picture about the upper part of an engine. OK, so you’ve got a read of 165 psi in a cylinder, what does that mean? most of the times , not a lot.
A more important check is a compression leak check which checks the whole upper part of the engine. It’s done by putting the piston you’re about to check in TDC (top dead center) when it’s at the end of the compression cycle, inserting a compression leak gauge into the plug and pumping air pressure in to the cylinder through it. The compression leak gauge will then give a read of the LEAK PERCENTAGE in that cylinder. Normal leak should be 2%-8% , but what’s more important , it should be the same with all the cylinders (unless you’re checking a single cylinder engine).
Another good thing with this check is that you can easily HEAR the air leak, thus pin point where the compression is leaking from. For example, if the exhaust valve is leaking you should hear a hissing coming from the exhaust end. If compression is leaking from the intake valve, you will hear air coming out of the air filter box, or if it’s the piston rings (or piston) that are very worn, you will hear air hissing from the engine case, coming out of the oil filler cap.