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Compression and How To Check It


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Keeping an eye on your compression is a good way to assess the condition of your top-end. Heat and wear gradually reduce the piston-ring and valve seal which keep the pressure in the cylinder causing power loss. Head gaskets eventually begin to leak from the heating and cooling cycles and can blow out unexpectedly. A simple compression test when your motorcycle is new will give you a baseline to compare future checks, but if you are already riding a higher mileage bike, check your service manual for the exact compression specs. Most big bikes run about 150 psi. or better. Stock cranking pressures for the Evo and Twin Cam are around 150 psi and 160 psi respectively. Here's the procedure for checking compression:

Remove both spark-plugs
Ground both leads so the coil or ECM isn't damaged
Install compression gauge, even cheap Car-Store gages work fine.
Hold the throttle wide-open
If you have a CV carb, hold the slide UP also
Crank the motor over until the gauge settle, usually 3 - 5 revolutions
If the reading seems low, squirt a tiny bit of oil into the spark-plug hole to give the rings a better seal. If the reading increases the rings are worn and cylinder maintenance is needed. If one cylinder is more than 10 lbs higher than the other, it may have a burnt valve, seat, or the ring/cylinder damage. Since each piston has separate compression and oil control rings, low compression doesn't always correlate with oil burning, but can cause fouled plugs due to incomplete combustion.

For you tool junkies there are several neat products for checking compression and leak down. Jim's Dual-Gauge Leak Down Tester enables you to check for worn piston rings, valves, and leaking gaskets by pressurizing the cylinder and enabling you to track down where the pressure is leaking by listening at the exhaust, intake, or crankcase. Motion Pro Compression Test Set features a conventional 300 psi pressure gauge with quick-disconnect adapters for 10, 12, and 14mm spark plug holes. Either of these work fine, but the Jim's Machine setup is the choice for pro workshops.

Bad compression isn't the end of the world, and your motorcycle may run for many miles as is, but for the best power and mileage it's best to bore the cylinders, replace the pistons and rings, and do a valve job.

Some other links on checking compression...

How to Check Your Engine's Compression - - Search Auto Parts | Automotive News

Engine Compression Testing

How to do a Compression Test

Properly Checking and Testing Engine Compression - Associated Content