After the valve clearance test, I wanted to test the compressibility of the motor. A standard compression tester can do this, but I think a cylinder leakdown test is more accurate.
In order to do this test the bike has to be at operating temperature, and the cylinder has to be at top dead centre of the compression stroke too, so a little planning is needed.
- Remove two of the rocker cover bolts before you start. It will speed up the disassembly process.
- Remove the fuel tank bolts, rad shroud bolts.
- Start your air compressor, set the regulator to 75-100PSI (or what your test kit says)
- Run the engine till its at operating temperature, then kill the motor.
- While the motor is running calibrate the leak tester till the needle is showing zero.
- Quickly disconnect the fuel line quick-connector on the left, and the electric fuel pump connection under seat.
- Remove the tank.
- Remove the spark plug.
- Remove the final two bolts on the rocker cover and lift up and out to the left of the bike (careful don’t lose the breather clip)
- Shift the motor into a high gear, 5th or 6th, then with the bike on a stand, rotate the rear wheel till the motor is at top dead centre.
- Connect the probe to the spark plug hole.
- Connect the tester to the probe.
Once the tester and probe connect, the pressure is released into the cylinder and the leakdown test needle will rise to show you the amount of leakage.
You should have time to re-calibrate the leak tester and re-do this test a few times before the engine cools.
For this bike, it started showing a 15% leakage with the tester set at 70PSi but quickly rose to a steady 60% when cranked up to 100PSI.
We already know that the valve clearances are in spec, so it’s most likely that the pressure is escaping either around the piston, through the piston ring-gap, or out bad valve seats. . we’re not quite there yet though :o)