A whole lot of problems all rolled into one!

[dbeardslee]

If you have leaking valves or broken rings the reading wont change much, if they all good the reading will be noticably higher.

The 'wet' test will always give higher results but if the results are only marginally higher the the "compression" is escaping through the valves (adding oil does not affect valve seating).
If it improves significantly then the rings are suspect.

OK, looks like we've got conflicting opinions as to what the wet test tells you. One says higher readings good, the other says higher readings bad. I lean towards Brian's explanation. If you've got broken or worn rings, wouldn't you expect them to let the oil go more easily past them, and so the readings wouldn't change much. If they were in good shape, the oil should act to fill the gaps and spaces between the rings and so the reading would go up. Question is, how do we know who's right?

[ManagerMike]

If one has worn rings, or more appropiately, worn ring-cylinder wall interface, then the 'wet' test will show a marked increase in the test results as the oil added for the test far exceeds the amount of oil present during operation and will therefore improve the sealing performance of said ring-wall interface for the duration of the test.
If one has broken rings, (or burned valves) well, a compression test wet or dry, will only tell you which cylinder(s) is stone cold dead, and is making all that godawful noise when the engine is barely running, by returnig near no compression readings at all, dry or wet.

[ManagerMike]

Let me add... under normal circumstances one does not perform a compression test on an engine that is running as expected...

[DGXSER]

In my experience, I have never thrown out a compression test for ambient air getting into the cylinders, Really, it takes several compression strokes before it produces the high reading anyway, and the only air coming in is ambiant. And I can assure you it DOES make a difference if you hold WOT or not. Try it out yourself, see what you get. It matters.

Now, as to what the readings tell you. If you get normal readings, count it done and move on. IF you get low readings, then add a couple teaspoons of oil to each cylinder and rerun the test ( add oil as you test each cylinder, not all at once). Again, it takes several cycles to get high reading. If the valves are the problem, the oil goes right past them and does nothing to help or very little. IF the number goes up substantially, then the XSive amount of oil in the cylinder has filled the gaps in the leaking rings, your rings are suspect.

I have only had one engine that sat for a long time and had low compression, Banshee, I have only got about 100 miles on it since I got it running. I have not rerun the compression on it yet. I have had several REALLY Knowledgable folks on this site that I have known for almost two years and highly respect them for their knowledge tell me the compression can come back from running them. I hope they are correct on this one.

[dbeardslee]

I did a little surfing around, and everything I found suggests that Mike was correct. This had a pretty good walk through and diagnosis. It's for a car, but still relevant.

[ManagerMike]

I have seen a greenhorn technician serving under me miss a stripped INTAKE rocker arm stud during a compression test because he saw compression on that cylinder (he botched the whole test). His missed diagnosis (that I had to correct) is what leads me to always instruct to disregard the first test so that the operation of all engine components are expressed in the results of the test, call it experience I guess.
As far as testing with WOT or not; it will affect the outcome as more air is allowed into the cylinders to BE compressed, however it is essentially irrelevent because the absolute value of compression is not as important as the relative value between the cylinders.

[crazy steve]

....As far as testing with WOT or not; it will affect the outcome as more air is allowed into the cylinders to BE compressed, however it is essentially irrelevent because the absolute value of compression is not as important as the relative value between the cylinders.

I'll mildly disagree with the above. While the 'relative value' between the cylinders is the most important part of the test, as this will indicate how smoothly the motor will run and point out any wear/damage issues in individual cylinders, checking at WOT isn't 'irrelevent' as this can tell you how worn the motor is overall. If a 'factory spec' is given, it's nearly always at WOT.

If you want a real eye-opener, check compression with the carbs removed; you'll get readings around 200 PSI...

'78E original owner

[b.walker5]

Every workshop manual I've ever read says that WOT is the way to do it. As for the meaning of the wet and dry tests, well I guess that will become a matter of opinion and we all know where that leads

I will say though that my uncle was a motor mechanic for 55 years and the 'wet' trick was learned from him. Was he wrong..?? Who knows ?? Cant ask him now, he's been dead for 28 years.

[DGXSER]

Well Brian, if your uncle was wrong, then so is Haynes, Clymer, and every other written source I have ever read on the subject. (Refer to pages 46-47 of the Clymer manual on the XS1100 for confirmation). That and over 20 years of my experience.

[dbeardslee]

If you want a real eye-opener, check compression with the carbs removed; you'll get readings around 200 PSI...

Just did one that way about a month ago. Checked compression on the warm motor when it pulled in the garage and got readings of 142 psi with very little deviation across the board. Checked again several hours later (cold motor) after reshimming the valves, and readings were 125 psi, again with very little deviation across the board. Both times compression was checked with the carbs off.

[crazy steve]

Just did one that way about a month ago. Checked compression on the warm motor when it pulled in the garage and got readings of 142 psi with very little deviation across the board. Checked again several hours later (cold motor) after reshimming the valves, and readings were 125 psi, again with very little deviation across the board. Both times compression was checked with the carbs off.

Interesting...

I checked the motor out of my 'parts bike' before mothballing it, and without oiling anything (I merely pulled the plugs and checked to make sure it turned freely by hand before spinning it with the starter), I got 205, 190, 125, and 195. Obviously #3 has some issues (I suspect valve seat seal), but I guess that says the other holes are in very good shape. I would be curious what you would have got with the carbs installed.

'78E original owner

[dbeardslee]

I've got to check the one I've got in the mancave so, just for you, I'll do it both ways (when I get that far) and let you know the results.