HOW TO CHECK THE CYLINDER COMPRESSION
First of all, you'll need a ball of yarn, three carrots, 8 fuild oz. of witch hazel, and magazine with at least 200 pages.
I kid. Read on:
#1) You'll want a good, fully charged battery. If you ride every day or trickle charge, you're golden. You'll be cranking quite a bit and you don't want your juice to run out half way through.
#2) Disable your ignition. Easiest way to do this is to disconnect the CDI box, which is the 2" x 3" x 1" black box located underneath the rear right faring. Just yank the big ol connector. This is not only a safety precaution, but it might save you a tiny bit of battery juice.
#3) Remove the spark plugs. Yep, all four. Do it.
#4) Screw in your compression tester gauge. XJ's use 10mm spark plugs, so it would make sense that you need a compression tester with 10mm threads. Most come with adapters for different engines.
#5) Hold the throttle wide open. This is to ensure you get the maximum amount of air into the cylinders.
#6) Press and hold the starter button. Let the engine turn over 6-7 times and then stop. Take a reading of the gauge and write down the results. Now let the air out of the gauge and do the test it again. Remember, wide open throttle. The second run should give you the same numbers. If not, do it a third time and write down the average of all three tests.
#7) Do #6 for the remaining three cylinders. Record your numbers.
The manual states that the cylinders should read 156 psi at sea level. If your numbers aren't exactly that, don't worry. Taking into account age and the fact that most people aren't at sea level, 145 psi is probably an acceptable minimum for a good, healthy engine. We're really shooting for consistency across all cylinders. Sure, they might not be 156 psi, but things wear out and age and it's not always going to be perfect.
For example, if you recorded:
#1 @ 145 psi
#2 @ 150 psi
#3 @ 148 psi
#4 @ 152 psi
Then you're in the green. You're good. The engine passed. Put it all back together and go on a victory ride.
But if you recorded:
#1 @ 145 psi
#2 @ 150 psi
#3 @ 148 psi
#4 @ 126 psi
That #4 cylinder is reading low. Let's find out what it could be. There are usually three things that cause a cylinder to have compression issues. One, the piston rings. Two, the valves aren't closing all the way. Three, the head gasket is leaking. Although when the head gasket is leaking, you'll usually see lower compression numbers on other cylinders, too. So how do we know if it's the valves or the piston rings?
Take about a teaspoon of engine oil and pour it into the spark plug hole. Engine oil. Not vegetable oil (although it might work, but don't do it). Not massage oil from the night before (although it might work, but really don't do it). Engine oil.
Run the compression test on that cylinder again. If the numbers improve, I hate to be the one to tell you that your piston rings are actin' a fool. They're not compressing the air properly. The oil sealed up the rings temporarily where the air was escaping and that's why the numbers got better.
If the numbers stay the same, then believe it or not, that is the better scenario. This means your valves are actin' a fool. Either your intake valve or exhaust valve (or heck, maybe even both) could be bent, or simply that you're way way overdue for getting your valve clearances in check. As time goes on, the valves beat themselves into the seat. It could be that it's hitting the valve shim and it's definitely, definitely time to address those valve clearances. Simple procedure, you got this.
What do I do from here?
Checking/adjusting valve clearances: http://xjrider.com/viewtopic.php?f=69&t=3529
The troubleshooting section, where you can ask for advice and help: http://xjrider.com/viewforum.php?f=9
