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Old 01-23-2012, 03:39 PM
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Default Yamaha 200 hp ox66 with a compression of 150 psi?

I have a friend who is looking at a 2003 Yamaha 200hp ox66 with 427 hours on it. The dealer has a folder for the boat which indicates the compression was checked in July 2010 and was (cylinders 1,2,3,4,5,6) 125,125,120,120,125,125. Had the compression checked on Friday and they report 150,150,150,150,140,140.

I called today with my concern about the compression reportedly going up so they checked it with 3 different compression gauges both hot and cold and got the same numbers.

What should I make of this? They say the plugs and cylinder walls have no carbon build up and the boat runs strong. We are supposed to go look at the boat hard on Saturday and he would like to buy it if this is not an issue.

Any help appreciated.
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Old 01-23-2012, 03:46 PM
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if there is oil in the cylinder it will raise compression by clogging the gap at ring/wall temporarily. Compression gauges do all read slightly different. Maybe this test was done with throttle wide open, unlike previous test? I have heard #s can jump with WOT vs Neutral.
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Old 01-23-2012, 03:49 PM
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retest
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Old 01-23-2012, 03:54 PM
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It's important that they are all about the same.
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Old 01-23-2012, 03:58 PM
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Quote:
Originally Posted by nickmo View Post
It's important that they are all about the same.
That's the most important thing. PSI differs greatly from gauge to gauge from my experience. Having 3 gauges back to back all reading the same is a good indicator.

I always do my own compression tests. If you're not able to do so, I would not take someone's word on it; I'd make time to be there in person to see everything with my own eyes.

It's too important to take someone's word on it, dealer or not.

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Old 01-23-2012, 04:18 PM
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Maybe the test with the lower ratings was performed with the throttle closed?
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Old 01-23-2012, 05:23 PM
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From what I have read on this site most say the compression should be in the 120's. I just wonder if this high compression means something. When you say they should all be about the same does that mean that having the two bottom opposing cylinders 10 psi lower then the rest a indicating a problem or something wrong?

Tomfl: Do you know the proper way to test the compression on this engine so I can verify how it was performed or check it myself?
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Old 01-23-2012, 06:17 PM
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I run a 250 OX66, compression is ~120. On my gauge, which is important to note.

Not sure if it's "the reccomended THT way" but what I've always done on a 2-stroke is to run the motor briefly to bring it up to temp, then remove all spark plugs, put motor in neutral and remove the safety shut-off clip. Install compression tester in cylinder #1, open throttle, crank motor until gauge stops increasing (usually 3-5 engine cycles) and write the # down on a sheet. Remove tester from #1 and move on down the line, writing each cylinder # down.

Spec is to have all cylinder #'s within 10%, the more even the better IMHO.

Again, there are probably variations on what I've listed above, but this has served me well in many, many outboard purchases and sales.
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Old 01-23-2012, 06:47 PM
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Quote:
Originally Posted by stringer bell View Post
... Maybe this test was done with throttle wide open, unlike previous test? I have heard #s can jump with WOT vs Neutral.
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Originally Posted by billinstuart View Post
Maybe the test with the lower ratings was performed with the throttle closed?
Won't make any difference; a 2-stroke will backflow through the exhaust port and fill the cylinder.
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Old 01-23-2012, 06:51 PM
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What's odd is that all of the readings are 150. Is this motor on a boat you can try?
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Old 01-23-2012, 07:16 PM
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Yes the motor is on a Cape Horn 19. Our plan is to go on Saturday and seatrial. Its about 4 hours away so would like to eliminate at least the obvious "bugs" prior to driving that far.
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Old 01-23-2012, 07:21 PM
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Quote:
Originally Posted by peace of mind View Post
Yes the motor is on a Cape Horn 19. Our plan is to go on Saturday and seatrial. Its about 4 hours away so would like to eliminate at least the obvious "bugs" prior to driving that far.
Run it and take compression with your guage. Those are great motors, as mentioned, the readings are not off between cylinders so I would not be too concerned at this point.
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