Low Compression in One Cylinder

Hello,
Im trying to get my 1978 XS400 running, i took a compression test and the left side was 145 PSI and the right was only 60 PSI. I put a little oil down the spark plug hole and it went up to 90 psi. What does this mean again?

Before I rebuilt my petcock i think it was flooding the engine, one guy told me the oil may have been washed of by the gas and thats why it raised the PSI. I tried this a few times and each time the compression would fall back down next time a tried, and go back up to 90 when I put in the oil.

Could this be that the valves need to be adjusted?
THANKS!

 

Oiling the cylinder helps the rings seal. Since the psi did not go up much that would indicate a valve problem. Check the valve adjustments the do the compression test again. A valve not closing all the way will make the psi low.
It is true that fuel can wash away the oil and ruin a set of rings.

 

How many miles on the bike? That will give us a better clue as well.

 

only 8000 miles on the bike, but it was sitting for awhile before i got it

 

With only 8000 on the clock, I would be looking at valves & head gasket.

 

Just exactly ... how long of "Awhile" ... did she sit?

If is sat for a long time, ... a Valve may be "Stuck" or a Face on an open Valve might have been exposed long enough to for Oxidation to have occurred, spoiling the Gas Tight Seal.

Look all around the Head for any evidence that the Gasket might be blown.

It would be worth taking-off the Cover and looking at the Valve Buckets on the Low Compression hole and see how they move when you jack the engine over.

 

since his compression went up by adding oil into the cylinder. that would be a sign of bad piston rings.

oil working it's way through the piston rings and coating the cylinder walls. thus, helping the rings to make a better seal.

put in a real thick oil. like 90 weight oil. your compression will probably jump even higher then 90 lbs compression.

but in his case where his compression is so low. it could be a combination of rings AND valves.

cylinder walls do have a tendency to dry out when sat for a long time. once he cranked it over. the dry rings probably scratched up the wall to an effect of low compression.

you probably don't have the tools to do this. but if you can come up with something to screw into the spark plug hole. like a compression gauge. without the gauge part. and hook it up to a compressor so you can blow air into the cylinder. put the cylinder on TOP DEAD CENTER ON THE COMPRESSION STROKE. where both valves should be seated. run compressed air into the cylinder. take the oil cap off where you put oil into the engine. if you hear air. you've got leaky rings. if you hear air hissing through the exhaust. you've got bad exhaust valve. if you hear air in the carb. you've got bad intake valve.

that's called a cylinder leak down test. to determine WHAT exactly isn't making a complete seal.

 

Your write-up is pretty darn good but I'd like to offer some additional details.
A cylinder leak-down test pressurizes the cylinder and, by means of a set of gauges, allows the user to observe the input pressure and then the rate of pressure loss in terms of percentage. More than 15% difference between cylinders (not unlike a compression test) is considered a possible problem. Listening for the loss of air in the mentioned areas is the next step but it is only part of a cylinder leak-down test. Harbor Freight sells a fairly inexpensive leak down tester here http://www.harborfreight.com/catalogsea ... own+tester . There are also several nice "how to use" write-ups available (I liked Car Craft's article).
Good luck getting that compression up, I know we'd all like to hear a success story here.