Performing a Motorcycle Compression Test

Motorcycle Parked On Dirt Road In Forest
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Even though a motorcycle engine may be running well, the internal condition of the cylinder may be deteriorating -- and you may not even know it. But can a classic bike owner with reasonable mechanical skills check the internal condition? Or is it best to leave it to the professionals and go to the dealership or a mechanic? Good news: There is a way to test motorcycle compression in the cylinder, and it's not all too complicated.

For an engine to run, it needs a fuel-and-air mixture under compression and a spark. In order for the engine to operate properly, all the phases must happen at the right time. If the mixture is incorrect or the spark occurs at the wrong time, or if the compression is low, the engine will not perform properly.

Checking the compression on a motorcycle engine is a very simple task. The tooling required is affordable and easy to operate to gauge the compression, and the results will tell the owner a lot about the internal condition of the engine. In short, a motorcycle compression test is possible...and simple.

DIY Motorcycle Compression Testing

A compression tester consists of an adapter to screw into the spark plug hole, a pressure gauge, and a flexible connecting tube.

To check the compression the mechanic will utilize the following steps:

  1. Warm the engine to operating temperature (this phase is not strictly necessary as the result will vary only slightly)
  2. Remove the spark plug, then replace it inside the plug cap and firmly attach the plug to a ground. Note that special care must be taken to ensure that the plug cannot ignite any fuel mixture that may be ejected from the engine when it is turned over at point five below)
  3. Screw the adapter into the plug hole
  4. Attach the pressure gauge
  5. Turn the engine over (either by an electric start or preferably via a kick starter if fitted)

As the engine is turned over, the movement of the piston will draw in a fresh charge, and this charge will be compressed after the valves (on a four-stroke) have closed. The resultant compression as the piston comes to TDC (Top Dead Center) will register on the gauge.

Every engine produced has different cranking pressure figures. However, most engines fall in the 120 psi ( pounds per square inch) to 200 psi. If the engine is a multi-cylinder, the pressure difference between the highest and the lowest recorded pressures must not be greater than 5 percent.

Typically, cranking pressure recordings will deteriorate over time as piston rings, valve seals and cylinders wear down. However, an engine that runs rich or consumes oil can create an unusual condition where the cranking pressure actually increases. This phenomenon (although it is rare) is a result of carbon deposits building up inside the engine (on the piston and inside the cylinder head) reducing the internal volume and thereby increasing the compression ratio.