1 cylinder low compression, or not?

[Guest]

I have a 1981 GS1000G. Runs pretty well but is a bit hard to start. I changed the plugs and that helped. Next was checking the compression. Cylinders 1 through 3 were low but consistent with each other: about 100psi dry, then went up to 150psi after squirting in a bit of oil through the spark plug hole.

1st question: the service manual spec says 100psi is the service limit but is that with or without the squirt of oil?

Bike was sitting for about a week since I last started it so I think that was enough time for the oil to completely drain into the crankcase. I also measured the compression cold without starting it and warming things up. So I would guess that when the engine is actually running the pressure was closer to the 150 psi oiled than the 100 psi dry, right? Meaning the rings are okay?

The weird thing was cylinder 4 (the right most). When I pulled this cylinders original spark plug it was very sooty with carbon, the other three looked fine. I changed the plugs about 70 miles ago, including two 30 mile highway runs at around 70+ mph. This brand new plug in the 4th cylinder was again pretty sooty with carbon when I pulled it to do the compression test though not as much as the original plug. When I ran the test, it only showed about 60psi. Squirted the oil and it had almost no effect, again about 60 psi.

Here's the odd thing: I realized that I had been measuring the compression without holding the throttle wide open (including for the other three cylinders' 100psi dry to 150psi with oil measurements, all with throttle closed). When I measured the compression on this 4th cylinder again with the throttle wide open, then it came up to 150psi like the other cylinders (this is after I squirted in the oil).

So is this telling me that the sooty carbon on the plugs and the low compression is due to the 4th carb not being synced or something like that? As opposed to being the rings? between the sooty spark plug and the odd compression behavior, I feel like there's something specific going on with number 4, I just have no idea what.

Thanks in advance for any help!

 

[uk gs nut]

Valve clearance check then sync carbs with gauges of your choice.

 

[Rijko]

1st question: the service manual spec says 100psi is the service limit but is that with or without the squirt of oil?

Without.

If that squirt of oil raises compression, it's an indication there is an issue with the piston rings not sealing properly.
That does not necessarily mean you have a problem : if the bike stood for a long time, the piston rings may be stuck
and will usually be freed during the first few hunderd kilometers.

Check out the "Top 10 Newbie Mistakes".
The biggest mistake is not doing the prerequisite maintenance while troubleshooting your problem.
That way you can exclude issues as contributing to the problem.

​So, FIRST adjust your valves.
Next, clean your carbs properly. See the links in my signature.
If you do not know if and when the intake boot o-rings have been replaced, do so now.

Only after that things like compression test or carb sync are done.

 

[unfocused]

Do a leak down test and this can give you a better idea of where it's loosing the compression. I suspect the valves were never adjusted, causing a tight valve and possibly coking it and no it's not sealing. I've torn apart 4 heads in the last year and found this to be true with all of them. And I'm not done. I have 2 more to do...