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post #11 of 135 (permalink) Old Nov 26th, 2009, 3:43 pm
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Engine compression tests @ 120 psi per cylinder, cold. So, it's not the valves being out of adjustment, leading to a lack of compression in one cylinder.
Throttle open or closed?

Compression should always be measured with throttle fully open!

If it was with throttle open, then 120 is far too low. 180 is more normal, even higher.

Figure 15*CR*1.2 or so as a ballpark.

Tom

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post #12 of 135 (permalink) Old Nov 26th, 2009, 3:52 pm Thread Starter
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OK, I put in some ordinary Champion plugs in place of the low resistance type and it ran a little better, but still stumbles. This may only be due to my cleaning them before putting them in, I don't know. I set the plug gap to 0.030". AFR is unchanged @ 11.5 to 1. The old plugs have black carbon on them, but clean at the spark gap. The vertical cylinder plug looks different than the horizontal one. The plug from the vertical cylinder has one side with some kind of black shiny coating on it, but the spark gap area is still clean. They look mostly like number 7 on this page:
http://www.dansmc.com/spark_plugs/sp...s_catalog.html
I guess the shiny stuff on one side is a little bit of oil? How can that be the case?
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post #13 of 135 (permalink) Old Nov 26th, 2009, 4:04 pm
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Leaky valve guides/seals?

Tom

1985 LeMans
1986 LeMans w/ sidecar
1992 XV250
1998 ST2
2003 SS1000DS
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post #14 of 135 (permalink) Old Nov 26th, 2009, 4:12 pm Thread Starter
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Throttle open or closed?

Compression should always be measured with throttle fully open!

If it was with throttle open, then 120 is far too low. 180 is more normal, even higher.

Figure 15*CR*1.2 or so as a ballpark.

Tom
Yes, of course. The throttle was wide open and the fuel injectors were unplugged. Do you usually measure it cold or warm?

Ah shit. I just measured it again, the engine was still warm. Vertical cylinder is 115 psi, horizontal is 110.

I wonder if running the engine with the old ECU that was way too rich caused excessive wear on the rings? I got the bike with 600 miles on it. It has about 5,000 now. I was religious about changing the oil.

Last edited by dirkwrong; Nov 26th, 2009 at 4:20 pm.
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post #15 of 135 (permalink) Old Nov 26th, 2009, 4:20 pm
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Yes, of course. The throttle was wide open and the fuel injectors were unplugged. Do you usually measure it cold or warm?
Cold.

But it could be your gauge. Do you have any previous readings with the same gauge? 120 just sounds really low . If I have time tomorrow I'll check my ST2 and SS and see what they give.

Tom

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post #16 of 135 (permalink) Old Nov 26th, 2009, 4:45 pm Thread Starter
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Cold.

But it could be your gauge. Do you have any previous readings with the same gauge? 120 just sounds really low . If I have time tomorrow I'll check my ST2 and SS and see what they give.

Tom
No, I bought this gauge specifically for my bike. For the metric guages, it's about 8 bar I'm reading. Thanks for checking yours if you have time. I've had enough for today. I'll put oil in the cylinders next time I test to see if the pressure comes up.
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post #17 of 135 (permalink) Old Nov 26th, 2009, 5:57 pm
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Quote:
Originally Posted by st2lemans View Post
Throttle open or closed?

Compression should always be measured with throttle fully open!

If it was with throttle open, then 120 is far too low. 180 is more normal, even higher.

Figure 15*CR*1.2 or so as a ballpark.

Tom
I understand the 15 as this is approximately atmospheric pressure in psi. Our CR is 9:1. What is the 1.2 factor?

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post #18 of 135 (permalink) Old Nov 26th, 2009, 7:29 pm Thread Starter
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According to this document, for a 9:1 compression ratio, we should see about 167 psi on a compression test. Man, this sucks! I HATE this. How could my engine be so far off?!??!?!
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File Type: pdf compression_testing_tutorial.pdf (25.7 KB, 121 views)
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post #19 of 135 (permalink) Old Nov 26th, 2009, 9:01 pm
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Before you panic, I would definitely try another compression tester. They are notorious for providing different measurements, especially the more inexpensive ones. Try to borrow a known accurate compression tester. A good two stroke engine builder (dirt bikes, snowmobiles, etc.) or a marina would be a good place to start. I'd be pretty surprised if your motor with that kind of mileage was suffering from low compression.

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post #20 of 135 (permalink) Old Nov 27th, 2009, 1:20 am
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I understand the 15 as this is approximately atmospheric pressure in psi. Our CR is 9:1. What is the 1.2 factor?
It's a fudge factor to take into account the additional pressure rise due to the increase in temperature, as explained in the document Dirk posted. The exact number depends on CR, valve timing, etc., again as explained in the document, but 1.2 generally works for the various engines I've measured over the years.

My Guzzis are normally 180-200, with a 9.5 CR or so, so I would expect similar numbers from a Ducati. And this is with open bellmouths and 25k miles since the last ring change!

Measure it cold, first dry and then with some oil and see what you get.

Tom

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2003 999R #29

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