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TOPIC: Compression
Compression 9 years 4 months ago #110656
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I have a late 80's (I think) 70 hp mercury. I turned it over and got 100 psi in all three cylinders. Is this OK?
Don |
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Re: Compression 9 years 4 months ago #110658
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it is poor at best I would say.
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Re: Compression 9 years 4 months ago #110662
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Agreed. But to be sure, try another gauge, it should be 120 psi across the board. If all are low but even it may just be that the gauge is reading incorrectly.
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Re: Compression 9 years 4 months ago #110673
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and was this a dry test.. or did you put some oil in the cylinders?
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Re: Compression 9 years 4 months ago #110674
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It was a dry test.
Don |
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Re: Compression 9 years 4 months ago #110675
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One good sign is that they were all the same compression. Besides variations between gauges, altitude and temp will effect the readings too. I live at 6500 feet and my 68 Merc 500 has 110 psi compression across all 4 cylinders. At sea level, 110 psi equates to a compression of about 134 psi. Hence, 110 psi is just fine for my motor.
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1970 Glastron V-142 SkiFlite
1965 Merc 500 1968 Merc 500 |
Re: Compression 9 years 4 months ago #110676
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well since that was a dry test.. those numbers will come up on a wet test with some oil in the cyl's..... i myself have seen cheap gauges have a noticeable difference in readings..i have been through about 6 different gauges myself..but i do this every day and might use one a few times a week..
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Re: Compression 9 years 4 months ago #110678
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I have been through 4 or 5 gauges myself. I now have 2. I use one for testing all cyl's, Then check one or two with the other for comparison.
Something I just learned here in this thread is elevation.  Just never thought of it. I live a 4,000 ft. I will take that into consideration now.
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Re: Compression 9 years 4 months ago #110679
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wet test only hides poor compression.
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Re: Compression 9 years 4 months ago #110681
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Now, I think I will reassess My figures. At 4,000 feet, I reduce my hp by 12% and for compression, I will ad 10% to "DRY" compression test. This should give me reasonable expectations for my outboards. May do better, but will be close.
Most outboards I have ran RPM checks on, fail to develop full RPM's by 4 to 6 hundred rpm. This is also, of course, related to boat and load, but I think it shows a trend, due to our elevation. You think?
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