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Brett's_daddy

50hp Mercury 2-stroke Compression Test

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Okay so i went boat looking again today to see a really sweet Crestliner Fish Hawk 1650. I really liked the boat and inspected it all over and thought it was worth what the guy is asking for it...until we came to the compression test for the motor. He had the motor running for about 15 minutes in a barrel of water. After he shut it off we looked it over and about 15 minutes later we did a compression test on it. We pulled all of the plugs (which were hot) and I screwed in the compression tester (rented from AutoZone), he then cranked the motor for like 4 revolutions and I got a reading of 70psi on cyclinder #1, I thought that was low so we moved to cylinder #2 and did the same thing and I got maybe a 75psi reading, on to cylinder #3 and got about a 72psi reading. I'm thinking these numbers are really low and should be almost twice what they are. I'm not 100% sure we did the test right...he didn't touch the throttle and I've seen that some people put the throttle wide open when they test...should we have done this? I really like the boat and if this test had shown 110psi in the cylinders I would have come home with it but these test readings gave me pause...I just couldn't bring myself to commit to the sale :(. The owner said the engine was professionally winterized last fall so shouldn't the marina have picked up on a compression problem if one exists? It really sucks, I want this boat but not if the motor has something serious wrong with it...can't afford a new motor! Of course all boat dealers around me are closed tomorrow so i can't pick their brains. What do you guys think...A.) did we do the test correctly (or should we have done it with the throttle open...or should we have done it right after we shut the motor off)and B.) did I do the right thing walking away?

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First thing, look into what it's supposed to be.  Next thing to look at, all the cylinders are within 5-10psi of one another which is really good.  What year is the motor and how many hours are on it?  If you really want it, call him up and ask him if he will agree to a test drive and put it on the water and see what it does.  Having all the cylinders so close in psi would ease some questions and concerns as none are way off of each other.

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From what I have read it will be low if the throttle is not open

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The motor is a 1997. I have searched many a message board and generally I'm getting that the readings should be 120 - 140psi...pretty much double the readings we got yesterday (although 105 - 115 wouldn't be horrible). He won't do a test drive as the boat isn't registered and apparently CT requires you have a valid boating license which he does not have.

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I don't think cranking the motor for only 4 revs is enough to build up pressure in the cylinder to it's proper test level.  But having them all read  similar is a positive sign.  

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Message boards are good for advice, but not what I would classify as a "reliable source". I would get a professional opinion.

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With them all being roughly the same, I doubt there is an issue. It was probably user error. I've never compression tested the 50 three banger but generally you want the throttle at WOT, no choke, and turn it over 10-12 times or until it stops building pressure. Having two people is handy, one to watch the gauge and tell you when its maxed out and the other to crank it. 

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6 hours ago, Brett's_daddy said:

The motor is a 1997. I have searched many a message board and generally I'm getting that the readings should be 120 - 140psi...pretty much double the readings we got yesterday (although 105 - 115 wouldn't be horrible). He won't do a test drive as the boat isn't registered and apparently CT requires you have a valid boating license which he does not have.

Well now you have a tool to get the price lower if he really wants to sell it.  And also take Nick B and Kevin22's advice as well if you go back for a second look.

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I think as others have said it's not the actual number as much as how close are the numbers relevant to each other. I have a 2005 merc 50 HP 2 cycle and mine are between 114 and 118 which I thought were low but I have a good friend in another state that is a mercury repairman and he told me this is fine. 

I have been told that Mercury doesn't publish compression numbers. I know that the factory service manual doesn't mention numbers. At least I cannot find them, the manual is disorganized somewhat. The Seloc book give a few random numbers that are not set in stone and the lowest is a 4/5 HP at 90 psi. Most are around 120. I would make sure you are using the tester correctly and that the throttle is wide open. You can kill the ignition by either pulling the dead man lanyard out or grounding the spark plugs to the block.

Personally I would look deeper into this and/or try again. If you haven't already check youtube there are a bunch of outboards being compression tested.

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Good advice here - I'd re-do the test after I made sure I knew how to do it.

What year is the boat?

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The boat is a 1997 Crestliner Fish Hawk 1650. The problem is this boat is a 3+ hour drive away from me so not easy to get back down to plus the guy did say he has other people lined up wanting to look at it (one that wants to trade him an expensive quad and cash). We were an hour late getting there (I did call and let him know we were running late though) but when we got there we went over everything for about an hour then he says he has to leave to help his father move some furniture and it was a 40+ minute drive away so we ran out of time...if he didn't have to help his father out I would have suggested we run the motor some more and do the test again but you know what they say about the best laid plans. I doubt that he still has the boat now but even if he did it's tough as we don't get out of work until 4:30pm and it's over a 3 hour drive away so by the time we got there it would be almost dark :(.

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Well you're learning, keep going and you'll find the one you're looking for and get a great deal.  Just be patient.

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