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post #1 of 26 Old 09-14-2009, 1:04 PM Thread Starter
 
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Compression test.

My bike is getting to the ripe old age of 25,000miles and being the paranoid person I am feels like she's lost a bit of grunt. The valves need adjusting along with all the other maintenence Ive done over the last summer (fork seals, head bearing, coolant flushes, HTEV work...etc wtc. But my main concern is the heart of the bike and most important part...the motor. So Im going to do a compression test this fall after I put it up for the winter, my question is, what does a compression test indicate, what does it tell you besides the obvious?
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post #2 of 26 Old 09-14-2009, 3:12 PM
 
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Re: Compression test.

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Originally Posted by temperedtim929 View Post
My bike is getting to the ripe old age of 25,000miles and being the paranoid person I am feels like she's lost a bit of grunt. The valves need adjusting along with all the other maintenence Ive done over the last summer (fork seals, head bearing, coolant flushes, HTEV work...etc wtc. But my main concern is the heart of the bike and most important part...the motor. So Im going to do a compression test this fall after I put it up for the winter, my question is, what does a compression test indicate, what does it tell you besides the obvious?
Nothing more than what what the PSI is in each cylinder, an overall picture. If you want more specifics, then you need to do a leak down test. But you don't do that until a compression check shows a problem.
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post #3 of 26 Old 09-14-2009, 3:50 PM
 
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Re: Compression test.

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Originally Posted by temperedtim929 View Post
My bike is getting to the ripe old age of 25,000miles and being the paranoid person I am feels like she's lost a bit of grunt. The valves need adjusting along with all the other maintenence Ive done over the last summer (fork seals, head bearing, coolant flushes, HTEV work...etc wtc. But my main concern is the heart of the bike and most important part...the motor. So Im going to do a compression test this fall after I put it up for the winter, my question is, what does a compression test indicate, what does it tell you besides the obvious?
lower PSI then specs = worn inside.
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post #4 of 26 Old 09-15-2009, 12:32 PM
 
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Re: Compression test.

unfettered is correct, a leakdown will give you more precise information. With a compression check, you're looking for a difference between cylinders more than actual psi. A cylinder with more than 5% difference than the others tells you something is going on. You need to do the valves before doing the compression check, too.
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post #5 of 26 Old 09-15-2009, 12:51 PM
 
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Re: Compression test.

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Originally Posted by cbr900racer77 View Post
unfettered is correct, a leakdown will give you more precise information. With a compression check, you're looking for a difference between cylinders more than actual psi. A cylinder with more than 5% difference than the others tells you something is going on. You need to do the valves before doing the compression check, too.
Good point about the balance (relative psi) between cylinders. Is it 5%? I was always under the impression that 10% was a standard limit.

However actual psi is important, you can have an engine with say 80 psi in all four cylinders and you would have a balanced engine with evenly, but badly worn rings. Granted it's still running smooth, but it's not making much power.
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post #6 of 26 Old 09-15-2009, 1:04 PM
 
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Re: Compression test.

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Originally Posted by unfetteredmight View Post
However actual psi is important, you can have an engine with say 80 psi in all four cylinders and you would have a balanced engine with evenly, but badly worn rings. Granted it's still running smooth, but it's not making much power.
That is possible but other things would become apparent. You would have excessive crankcase pressure with oil pushing itself out at seals, gaskets and/or the airbox. Milkshake mess everywhere it leaks out. I call that a tired engine.
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post #7 of 26 Old 09-15-2009, 1:09 PM
 
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Re: Compression test.

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That is possible but other things would become apparent. You would have excessive crankcase pressure with oil pushing itself out at seals, gaskets and/or the airbox. Milkshake mess everywhere it leaks out. I call that a tired engine.
Well it was a very low amount to just make the point. I would be just as concerned if the readings were all 120-130 psi considering they're supposed to be around 170.

Milkshake mess, that's amusing.
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post #8 of 26 Old 09-15-2009, 2:11 PM
 
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Re: Compression test.

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Originally Posted by unfetteredmight View Post
Well it was a very low amount to just make the point. I would be just as concerned if the readings were all 120-130 psi considering they're supposed to be around 170.

Milkshake mess, that's amusing.
I understand but it would do it at probably 120 psi as well. Just less of it. Once the PCV system has been overwelmed with excess pressure, it starts to create a positive pressure in the crankcase and will slobber somewhere.
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post #9 of 26 Old 09-15-2009, 4:28 PM
 
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Re: Compression test.

You're right, 10% is the industry standard but I start looking for problems at 5%.
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post #10 of 26 Old 04-12-2016, 10:59 PM
 
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Re: Compression test.

Finally found what my compression is supposed to be!! Thank you guys. So quick problem for you guys and a follow up question. I'm blowing white smoke when I throttle hard. My engine was pretty oily, I cleaned it up. Took my compression. 148,170,150,170. In that order left to right. My forth spark plug had oil in the top section they are all black sooty even a ashy colour at the tip. I ordered a new gasket. But while I'm taking out my engine and head how do fix my compression on a budget? Valves and seals? Do I do all four even though two are good. Cheers
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post #11 of 26 Old 04-12-2016, 11:36 PM
 
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Re: Compression test.

Do all work evenly. Brakes, Cylinders, Valves, etc....

To be Old and Wise, You must first survive being Young and Stupid!
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post #12 of 26 Old 04-12-2016, 11:53 PM
 
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Re: Compression test.

Makes sense. I'm gonna hold off then. Put the gasket on. That may help anyway. And do my valves over the winter. Thank you
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post #13 of 26 Old 04-13-2016, 9:56 AM
 
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Re: Compression test.

I now have a low of 148 and how do I determine the cheap way out?
I now pour a little bit of oil down the plug hole.
I take another compression reading cold so I have a cold gap, not a hot gap at the rings as heat will close them some.
A. No change in psi = Valves leaking.
B. Rise in psi = Rings sealed with the oil to tell who is leaking the most, aka, in the cheap seats leakdown test.

Cold vs. Hot Comp Test:
Hot = I do not walk out to a running car/bike the day it does not start is that day.
Cold = I only test cold compression tests just for that reason is now how much kinetic are we talking?

Signed,
NOLTT (no one listens to turtle)
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post #14 of 26 Old 04-13-2016, 10:13 AM
 
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Re: Compression test.

Smart! Those were cold test numbers. By the time I read the running temp part I had drained my coolant and had engine degreaser all over half my engine. I'll give your method a try. Thanks
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post #15 of 26 Old 04-13-2016, 11:00 AM
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Re: Compression test.

Did you have the throttle wide open when you did the test? If not then the numbers are not valid.
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