Low compression, bad piston rings? How to test.

[spankyb11]

Hey guys,

First off, I have an 88 K75 S with 77,000 miles on it. When I bought it, the bike had zero compression in cylinder two, which I traced to a blown exhaust valve. I replaced the valve. Now I have really low compression on the same cylinder. The bike rides fairly well. I am just worried about any further damage, as I suspect a larger issue is what caused the valve to blow initially. My compression is as follows; Cylinder 1 - 165 psi, 2 - 120 psi, 3 - 160 psi. After replacing the valve, I did adjust the valve shims, so all of the clearances are in spec.

My questions:

1. What kind of damage will come from these numbers if left unfixed? And how urgent is this to fix? Can it wait until riding season is over, or should I park it until I solve the issue?

2. I am wondering what test can be done in place of the wet compression test to see if the piston rings are sealing properly? The manual, as well as my intuition suggests that a wet test would be pointless since the cylinders are on their side. I did attempt to perform this anyways, and my numbers didn't change from the dry test. I am aware of a leak down test, but I don't have access to a compressor. Any other test options?

3. Am I right to suspect the piston/cylinder wall seal? I am getting a wet oily buildup on the spark plug.

Thanks.

[Martin]

 Try doing a wet compression test. Could possibly be sticky rings can happen if it has been left siting for a long time. You can try running a high detergent oil, or sometimes it will free up with use.
Regards Martin.

[spankyb11]

Hey Martin, thanks for the reply. I did attempt a wet test, but since the cylinders are on their side, the oil didn't form a complete seal and the reading was identical to the dry test. I then looked it up in the manual and it says doing a wet compression test is pointless and results will be inconsistent due to this fact. I know the Haynes manuals aren't always the best, but I wasn't able to get a complete seal around the piston during my test. Is there a trick to getting the oil to form a seal on the sideways cylinders, or is there another test that can be performed that gives the same information?

[kmev]

Did you replace the valve seat, too? That was the real problem.

[Martin]

You can also perform a leak down test, special gear and a compressor is required. The gear can be bought or if you are handy made. Or get a couple of strong mates and tilt the bike as far over as you can, use a pressure pack can of oil and spray up through the plug hole aim high. Give the bike a couple of cranks to coat the bore and rings, you might have to try it a couple of times to get a reading. If you have an old mattress I've heard of guys lying the bike right down, but you would have to remove the fairing and have a sealed battery.
Regards Martin.