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.
Note: Some dirt bikes may require some disassembly, such as removal of the fuel tank so as to be able to screw the hose into the cylinder head at a straight angle and not kink the hose or damage the soft aluminum threads dropping shavings into the cylinder. If you can't easily thread the compression tester into the spark plug hole, be sure to remove anything in the way so as to allow access.
Before You Perform A Compression Test...
If It's a
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.
- 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 (Available at Auto Parts Stores) 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.
Note: If you're working with an electric start motorcycle, you'll likely need to leave the engine run switch turned to "ON" so as to be able to use the starter for turning the engine over, just be sure to disable the ignition system by disconnecting the primary lead from the ignition coil (small single wire or weather proof electrical connector) and Waterproof the Electrical Connections at reassembly.
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 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.
After finishing up with the compression test, (provided that the pressure reading looks good), replace the spark plug with a new one of the correct type and heat range, then put everything back together and check out these:
Dirt Bike Maintenance Articles