How To Compression Test A Dirt Bike

Performing a compression test on a dirt bike's engine can tell a lot and it's something anyone who rides should do occasionally as this gives you a better feel of the engine's top end condition than "feeling the kick", Doing this can serve two purposes. First, it can give you an indication of how close you are getting to needing a top end freshening, secondly it can tell you whether or not any problems you may be having with the performance of the dirt bike such as hard starting, or low power are related to low compression.

This is a pretty simple and straight forward procedure as you'll see below, so whenever you freshen up the top end on a dirt bike, or have a few hours on the top end, it's a good idea to perform a compression test, as well as after breaking in a new set of piston & ring(s), then Record the Reading by scratching it in yourself in pen on either our Advanced Setup Log
or
Basic Setup Log

In the future, when you check the compression readings, if the pressure drops 20% from the reading that you noted after the break in period of a fresh top-end, this would indicate that the motor likely needs a tear down to check the condition of the top end, which includes the piston, piston ring(s), exhaust valve with it's related components on a 2 stroke, or the cylinder head and valve train on a 4 stroke, as well as the cylinder's condition on a 2 or 4 stroke.

Before You Do a Compression Test...

• Make sure the dirt bike is clean It's important there's no dirt or water near the spark plug or coil pack on 4 strokes otherwise dirt or grit could get into the cylinder upon removal of the spark plug which is never good and leads to problems such as scoring of the cylinder walls and accelerated wear of the piston & rings, among other parts not mentioned.
• Turn the gas off at the fuel petcock.
• Ensure that the air filter is clean and in good condition.

• If necessary, remove the fuel tank as this will allow you unobstructed access to the spark plug, and you'll be able to thread a Compression Tester into the cylinder head without any sharp angles being placed on the hose, as well as allowing you to inspect the cables and hoses for any under tank chafing or other damage.


• Finally... Remove the spark plug, and using the appropriate hose or adapter with the appropriate thread size and pitch on the compression tester's hose end to mate the threads in the cylinder head, thread the compression tester hose into the spark plug hole and tighten it finger tight, or just enough so that the o-ring or gasket which seals the compression tester to the cylinder head is making contact and becomes slightly compressed.

Performing the Compression Test

• For the following steps and assuming everything above has been covered, hold the throttle wide open with the engine stop / kill button depressed, or have the engine run switch turned to OFF.

Next, with the throttle held wide open, Kick the engine over, (or by using the electric start as can be found on some models) turn the engine over until the needle on the gauge of the compression tester peaks at it's maximum pressure and record this reading for future reference.

The ideal pressures for an average 2 stroke dirt bike at sea level that is not heavily modified should be in the vicinity of the numbers below.

• A 50cc, 60cc, 80cc, 100cc, 125cc 200cc or 500cc should be able to squeeze out a reading of 120-190 PSI.
• A 250cc should be in the neighborhood of 170-240 PSI.

If you're unable to reach a satisfactory compression reading, or there's a 20% difference (or more) from the reading you obtained after breaking in a new top end, this may be an indication that the top end is in need of being rebuilt.

If it's determined that the top end IS in need of being rebuilt, you really should pick up a factory service manual specific to your dirt bike if you don't already have one, then disassemble the top end to check the condition and clearances of parts as detailed in the factory service manual, replacing parts that are worn beyond the service limits stated or have a clear indication of wear as well as at a minimum, always replacing the piston ring(s) after disassembly no matter of their physical appearance or measurement.

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