Maybe you’ve had your bike a long time and the odometer is showing high mileage, or maybe it’s a recent, low-mileage acquisition, but you’d like to know if the engine is in top running condition. Among other things, like proper carburetion and spark, engine performance is directly related to the cylinder’s capacity to generate proper compression.
Every engine needs compression so that the air/fuel mixture will burn with enough force to push the piston back down again. Low compression will cause a weak burn, which reduces power output and increases fuel consumption. In a four-stroke engine, compression is generated after the air/fuel mixture has entered the cylinder during the intake stroke, the intake valve (or valves) has closed and the piston begins its upward compression stroke. As the piston goes up, it squeezes the air and fuel mixture into the combustion chamber, where at the proper time, a spark ignites the compressed mixture.
There are several areas where a cylinder can lose compression. Compressed gasses can escape past worn piston rings, worn cylinders, poorly seated or burnt valves, and past an inadequately sealing or damaged head gasket. In extreme cases, it can also escape through a cracked cylinder head.
There are a couple of ways to check how well a cylinder seals. The most basic, which can be done if a bike refuses to start and you know that fuel is getting into the cylinder and the spark plug is sparking, is to place your finger over the spark-plug hole of a cold engine and feel for compression while cranking. Note that any time the engine is cranked to check compression, all spark-plug leads must be disconnected and spark plugs removed, inserted into the plug wires and grounded so that they spark, to prevent the ignition coil from overheating. Ground the plugs at a location away from the spark-plug holes, so as not to unintentionally ignite the fuel mixture blowing out of the open cylinder. The finger check is a very rudimentary verification that will indicate a major problem with the engine if very weak or no compression is felt. If there’s enough pressure to lift your finger off the hole while cranking, it should be enough to start the bike.
A more thorough verification is a proper compression test using a compression gauge. Specifications vary, so you will need a service manual to know what compression reading is acceptable for your bike – the maximum reading for my KLR650 is 129 psi, while the minimum reading for my girlfriend’s SV650 is 156 psi.
You follow the same procedure as you would with the finger test, except this time the engine should be at operating temperature, and instead of using your finger, you screw the compression gauge into the spark-plug hole. Also note that when cranking the engine, you should open the throttle wide to allow maximum air into the cylinder.
An indication that all is well, is that at the first upward stroke of the piston, when the gauge needle first moves, the reading should be about 70 to 80 percent of the maximum reading. If it takes a lot of cranking to reach the minimum specified reading, the engine will run, but it will not perform optimally. And, of course, if you are checking an engine with more than one cylinder, all of the cylinders should be within specs.
If the reading is lower than specified, you can narrow down where the compression is being lost by squirting a few drops of oil into the cylinder; if the compression rises after this, the piston rings are most probably the culprits, as the oil would help seal the rings. Compression can also be checked with the engine cold, and if the reading is within specs, it should normally improve with the engine warmed up, as the piston rings will seal better.
Should your engine have a low reading, an instrument called a leak-down tester will help pinpoint where the engine is losing compression. A leak-down tester forces compressed air into the cylinder, through the spark-plug hole, and measures the amount of air loss, giving a reading in percentage points. Some loss is unavoidable, as there are gaps in the piston rings that allow for their expansion, and even well-seated rings do not match perfectly to the cylinder wall.
A leak-down test is also performed on a warm engine, and it is done with the engine off and the piston of the cylinder being tested placed at top-dead-centre (at the very top of the cylinder) with the valves closed.
An engine in good condition can have a loss of up to 15 percent (race engines are usually kept below five percent), and anything below 10 percent means the engine is in optimum running order. On multi-cylinder engines, all cylinders should be within a couple of percentage points of each other.
If the test reveals a loss of more than 15 percent, it doesn’t mean it’s time for a rebuild, though oil and fuel consumption might be higher than normal. A loss of more than 20 percent means it’s probably time to open up the engine. If such a loss is experienced, you can listen for the air leaking past whichever component or components may be worn out. A hissing coming from the tailpipe means the exhaust valves are not sealing properly. Hissing through the carburetor or throttle body means the intake valves are not seating properly. A hissing though the engine breather hose (usually plugged into the airbox) means it’s time for new rings or a re-bore.
Moderately high oil consumption is not always a sign that an engine is worn. My KLR uses about a half litre of oil per 1000 km, yet the engine has less than five percent loss at the cylinder, and the spark plug is completely clean.
The leak-down test is the ultimate tell-all as to the condition of your engine’s cylinders. Any well-equipped shop or mechanic must have one, as it is a valuable diagnostic tool that can either prevent or confirm a teardown. And any home mechanic should have a compression gauge handy. It, too, can help you find a problem should your bike refuse to start.
Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.
