Where did the Compression go????

[michael]

I have a 81 GS850L parts bike I bought to help get my 80 GS850L on the road. The motor made the usual " da da da Daaaa da da da daaaaa" sound when cranking that let me know that one cylinder had bad compression. Sure enough, when I did a test, cylinders 1 to 3 had around 130 to 140 PSI, but #4 had about 20 to 30. I had seen this before on another bike, and it had a dime sized hole in the piston when I disassembled it.

I disassembled this motor and I don't see anything wrong!!! The piston and rings are fine. The head gasket is ok. The cylinder looks perfect and the head looks ok. The valves look to be seated up in the head properly.

Maybe the valve seats are bad and I can't tell from looking?? This is the only thing I can think of.

I want to try to sell the head to fund my tight budget GS850L project, but I can't if I suspect it is no good....

Anyone have any ideas??

Thanks,

Mike.

[flyingace]

Do you have adequate valve clearance on #4? A valve that doesn't close all the way will cause very low compression.

Did you dump some engine oil down the plug hole of #4 and retest? If the rings are bad, this will bump up the compression a noticeable amount.

[2ndstage]

Do you have adequate valve clearance on #4? A valve that doesn't close all the way will cause very low compression.

Did you dump some engine oil down the plug hole of #4 and retest? If the rings are bad, this will bump up the compression a noticeable amount.

This is always a good test if you want to rule out the bottom end.

[michael]

Thanks for the quick responses!

I did not put any oil in the cylinder and try it again. I thought with such low compression something catastrophic had happened. The engine only had 17K miles on it. If the valve had worn down that much, it must have begun to be noticeable to the previous owner before it got so bad that the compression was that low?

I know what I will do next time though... before I tear it apart!

Thanks again,
Mike.

[hungryman]

Yea, if you run through the troubleshooting to the end, then you'll have it narrowed down much better.

Like they said, a squirt of oil in the cylinder - if the compression jumps up, then it's the rings, if not it's the valves.

If it's the valves, loosen and adjust them on the cylinder in question. If they are too tight the compression will be low.

After that, take the wrench to it if the compression doesn't improve. You'll have a good starting point.