Know Before You Invest
In this video, Bob takes us through a compression test on a Twin Cam motor.
The motorcycle in this video has sustained damage due to an accident. It is already known that the frame will need to be replaced.
It is, also, important to know the status of the motor before moving forward with the overhaul of this motorcycle. It would be very wasteful to continue with the removal, and reinstallation of this motor if it has underlying damage.
A motorcycle compression test is an important starting point to get a good view of the health of an engine.
Start your test by removing both, your front and rear spark plugs. Install a compression gauge onto your front cylinder. Hold your throttle wide open to allow for as much air as possible to enter the engine for its compression stroke, when the engine is turning.
Now, you can turn on your ignition switch and crank the motor over a sufficient amount of time, until the indicator in the compression gauge stops climbing. Record your reading. Repeat these steps for the rear cylinder.
Reference Your Manual
It is important to refer to your service manual to know what the compression range and variance should be for your particular engine.This engine, for instance, should have no less than 90 lbs of compression and the cylinders should not be more than 10% apart from one another.
If you suspect that your motor is worn, or damaged; consider a motorcycle compression test before moving forward and disassembling. A compression gauge is a helpful tool to use to diagnose or gauge the health of your engine.