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.
If this is a 4 stroke dirt bike we're working on, it's not going to be possible to base your readings off of the suggestions listed below.
Due to the auto decompression release mechanism found on 4 stroke dirt bikes, you're going to need a service manual specific to that dirt bike as you can then obtain the true recommended pressure by the reading to determine the condition of the top end on a 4 stroke.
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.
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 should get a service manual specific to the dirt bike, and / or refer to our article on rebuilding the top end on a 2 stroke, 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 replace the piston ring(s) upon disassembly no matter of their physical appearance or measurement.