Compression Test

[GtrMn]

Just did a compression test after doing one 8k miles ago. last time all the cylinders were over 155. The bike now has 20k on it. this time....the test came out 160...120...105...150. Added some oil to the #3 cylinder and the compression went to 180! That would suggest a ring problem...BUT...the bike runs great...starts right up with no choke...42+mpg. Smooth and uses almost no oil at all between oil changes (1k miles) and the plugs look perfect. That makes no sense at all to me!

[prometheus578]

That makes no sense at all to me!

(probably one of a long list of things that don't make sense to you...)
Be that as it may...
Could be a time delay and a weakened battery. A slowly cranking engine will give lower readings. What I also mean is... Some people find that their compression readings go down across the cylinders as they test..(as the battery gets weaker)
(as I know that you didn't have oil handy when doing the test, there's the possability that the battery recouped a little as you searched for your oil can... resulting in a little higher reading, which you attributed to the squirt of oil)

Don't condemn the cylinders yet.
Recharge the battery and try again.
All plugs removed, throttle held wide open, etc.
Post your results.

[GtrMn]

resulting in a little higher reading, which you attributed to the squirt of oil

It went from 105 to 180...yow!

[fredintoon]

only part of the story

Hi GtrMn,
you need three tests, not just one.
1) Compression test
2) Leak-down test
3) Road test.
3) is the important one. If the bike runs good, ignore 1) & 2)

[GtrMn]

3) is the important one. If the bike runs good, ignore 1) & 2)

Haven't done the leak-down test yet....I like that part of the quote though....cause it runs like a champ!

[red bandit]

If the bike runs good and doesn't use oil........what's the problem.....ride it.

[vincer]

So to do a proper compression test, pull ALL spark plugs out and hold the throttle wide open?

[vincer]

Answered my own question

Never mind. I figured it out.

[GtrMn]

Gonna do the compression test cold tomorrow..may do the leak-down test too. Oh BTW if you happen to have a cruise control...don't forget to turn it off before you start it back up....oooops scared the crap out of me!

[ratbyk]

Compression test should be done hot not cold. Engine should be up to full operating temp.
Remove all plugs.
Hold throttle wide open.
If you do add any oil to the cylinder, use just a small amount. Crank engine over to distribute the oil around the cylinder and disperse any extra oil to get a truer reading.

[GtrMn]

Thanks Rat...knew that though...just wanted to see what the difference is.

[GtrMn]

Leak-down test

What's the easiest way to know when each piston and valves are the right position for this test? And, should it be done with the engine warm?
Thanks in advance!

[prometheus578]

Engine should be warm.
Take the valve cover off so you can see the valve train.
Using a wrench, crank the engine over till it's at TDC. By looking at the valves, you can determine which cylinder(1 or 4) is on the compression stroke. Check that one, then turn the engine over to TDC again and check the other one.
For #2 and #3, I don't recall if there's a mark on the timing plate for them. Look down their spark plug holes, watching the piston come up as you trun the crank. When It's at the top of it's travel, check the cylinder that has it's valves closed. Repeat, like you did with #1 and #4.
The wrench that you use to turn the crank over... be sure to remove it from the crank before you pressurizing the cylinder. Crank shafts have been know to spin when using compressed air, launching the wrench who knows where!

[TopCatGr58]

Hey Tom/Gtrmn,

I don't like having to remove my valve cover if I don't have to. You've got a compression tester, so....use it to determine the compression stroke on the inner cylinders, or all other three besides #1.

Use the "T" mark on the timing plate to find TDC for #1, test its leak level. Then put your comp. test into #2, and rotate the engine until you see the pressure level rising on the gauge. STOP, remove the gauge, and then either visually or use a long straw to help find the TDC position for that cylinder, and then test it, and then continue with the other cylinders. IMHO, easier than removing the valve cover!

Also, like Prom said, you'll want to put it in gear, and wedge the rear wheel so that the air pressure can't turn the engine over!

If you're confident in your compression testing technique and values, then there is probably some thing(s) wrong! The stock comp value for a non-modified XS11 is 142psi + 14 at Sea Level. So.....you've got 1 cylinder a bit over spec, 2 below and 1 just about right. As has been stated, carbon in the chambers can cause problems like detonation due to irregular dome and piston surfaces, and hot spots that can cause preignition. Also, the comp levels shouldn't be more than 10% variation between all cylinders. Uneven crank stresses can occur with too irregular values!

Your oil spritz test did hint towards worn rings vs. valves. I can't remember how long you've had the bike, what the history of it was, how many miles you've put on it since, but some problems can be solved by putting several hundred highway miles on it, burning out the carbon, reseating the rings, unsticking possibly stuck rings,etc. IF you decide to try to remove the carbon, with the chance of it having stuck rings, the MMO treatment would be good for both the carbon, valves as well as possibly help seep around to the rings to free them up also. Adding a few ounces to the engine oil after the top end soaking, running the engine ONLY ON THE CENTER STAND, NO LOAD for about 5 minutes after reaching operating temps(put fan blowing across headers into engine to keep from overheating), and then drain oil and replace filter/oil with fresh, and then take it for an extended ride with some well placed high rpm bursts! Then bring it home and retest your comp levels! You may be pleasantly surprised afterwards!
T.C.

[GtrMn]

A leak down test is done by applying low pressure air to the cylinder, with the piston at the bottom of it's travel. If you hear air escaping, either through the airbox, exhaust or the oil filler hole, that will locate the compression loss. You can do this with a compression gauge that has a detachable end, and use an air compressor. The air coupler is usually the same on both items, the air hose and the tester hose. Use low pressure to perform the test. There may be a tech tip here on the subject.

Found this from another post. Does this work also? It would mean not having to put the bike in gear and not needing to block the rear wheel anyway. May try the MMO also. Oh, does Seafoam work like MMO?