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Jan 23rd, 2013
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Buying a used boat ..... #13531137 04/23/20 01:24 PM
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jayhdz78 Offline OP
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Guys , wanted to ask when looking into buying a boat especially a little older 95 model , what is the easiest way to do the compression test ? And what should it read ?

Last edited by jayhdz78; 04/23/20 01:27 PM.
Re: Buying a used boat ..... [Re: jayhdz78] #13531157 04/23/20 01:39 PM
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ssmith Offline
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on any used boat take it to the lake an run it if it doesn't run right tell the guy to fix it an call you .on a over 20 year old boat you will have some repairs in the future.

Re: Buying a used boat ..... [Re: ssmith] #13531203 04/23/20 02:03 PM
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jayhdz78 Offline OP
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thanks

Re: Buying a used boat ..... [Re: jayhdz78] #13532302 04/24/20 04:21 AM
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Compression test is done by taking all the spark pugs out and then spinning the motor over with the starter while holding a compression test gauge in one spark plug hole at a time until it gets its maximum reading. Do this individually on each cylinder then compare the readings

The number could be significant, but as long as all of them are within about 10-15% of each other that would be about a normal reading. Gauge to do this is less than $20.

Hours are probably not available on a 25 year old outboard.


Luck will beat skill any time you have enough of it!
Re: Buying a used boat ..... [Re: jayhdz78] #13532344 04/24/20 10:20 AM
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Lots of things to look at! I fish out of a 96 model bass boat every weekend. I take great care of it so it serves me well. Just do a good look over at the whole boat. Motor compression is important. But look at the hull, transom especially. Look for cracks as some boats are prone to wood rot. I don’t know your deal your working but there are some great older boats out there! But there are a lot of boats you need to walk away from. Good luck!

Re: Buying a used boat ..... [Re: jayhdz78] #13532682 04/24/20 02:56 PM
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jayhdz78 Offline OP
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Great Info .

Re: Buying a used boat ..... [Re: jayhdz78] #13535951 04/27/20 01:40 PM
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Harbor freight sells a compression kit for something like $40. Just pay the extra to get the “professional” version since the components are better, since you need good seals to get an accurate reading. You may also want to do a leak down test too. YouTube is a great resource for step by step compression check.

When buying a used boat, check the maintenance on the fuel and pump systems. Those go out and are hard to diagnose.
Tim


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Re: Buying a used boat ..... [Re: jayhdz78] #13536076 04/27/20 03:08 PM
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As others have already stated the least you can do is a compression test . I looked at a very nice boat and was ready to pull the trigger without hassling around with testing compression . I finally decided to check and one cylinder only had 15 lbs compression and the other 3 barely 60-75 . I double checked the gauge by hooking it up to my car and the gauge was working correctly . I'm glad I checked .

Re: Buying a used boat ..... [Re: jayhdz78] #13536360 04/27/20 05:44 PM
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Compression numbers vary depending on the motor, but generally they are anywhere from 100-130psi. And like already said, within 10-15%. I honestly prefer that they are all exactly the same. Also take a good look at lower unit, and see if they will let you check the gear oil for water. As for the boat, check floor and transom. Tilt the motor up a bit and push down on the bottom of the motor, check the transom for flex when you do this. If it moves, then you move...on to the next boat that is....

Re: Buying a used boat ..... [Re: PlanoPaul] #13536440 04/27/20 06:33 PM
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Originally Posted by PlanoPaul
Compression numbers vary depending on the motor, but generally they are anywhere from 100-130psi. And like already said, within 10-15%. I honestly prefer that they are all exactly the same. Also take a good look at lower unit, and see if they will let you check the gear oil for water. As for the boat, check floor and transom. Tilt the motor up a bit and push down on the bottom of the motor, check the transom for flex when you do this. If it moves, then you move...on to the next boat that is....

Did you know that some engines are intentionally designed with different levels of compression on different cylinders? Because of this, the numbers are meaningless without comparing results to the specifications for the particular engine being examined. I have a V6 Evinrude that calls for a different PSI on the bottom two cylinders as compared to the top 4. Without consulting a service manual, you might start thinking there was something wrong with that engine when there really isn't.

Re: Buying a used boat ..... [Re: Flippin-Out] #13536564 04/27/20 08:01 PM
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Originally Posted by Flippin-Out
Originally Posted by PlanoPaul
Compression numbers vary depending on the motor, but generally they are anywhere from 100-130psi. And like already said, within 10-15%. I honestly prefer that they are all exactly the same. Also take a good look at lower unit, and see if they will let you check the gear oil for water. As for the boat, check floor and transom. Tilt the motor up a bit and push down on the bottom of the motor, check the transom for flex when you do this. If it moves, then you move...on to the next boat that is....

Did you know that some engines are intentionally designed with different levels of compression on different cylinders? Because of this, the numbers are meaningless without comparing results to the specifications for the particular engine being examined. I have a V6 Evinrude that calls for a different PSI on the bottom two cylinders as compared to the top 4. Without consulting a service manual, you might start thinking there was something wrong with that engine when there really isn't.


Well I have read about there being a 5 psi difference between the starboard and port cylinders on some Evinrude and Johnson’s, but I have also seen them be exactly the same. Not being a trained technician, I don’t always no what to believe or expect. What motor do you have? I would like to know the purpose of that if that is indeed the case.

Re: Buying a used boat ..... [Re: jayhdz78] #13536592 04/27/20 08:20 PM
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You can "borrow" a compression test kit at O'Reilly's Auto, by leaving a deposit. I'd go there with the boat and do it in their parking lot! IslandJim


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Re: Buying a used boat ..... [Re: PlanoPaul] #13536643 04/27/20 08:48 PM
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Originally Posted by PlanoPaul
Originally Posted by Flippin-Out
Originally Posted by PlanoPaul
Compression numbers vary depending on the motor, but generally they are anywhere from 100-130psi. And like already said, within 10-15%. I honestly prefer that they are all exactly the same. Also take a good look at lower unit, and see if they will let you check the gear oil for water. As for the boat, check floor and transom. Tilt the motor up a bit and push down on the bottom of the motor, check the transom for flex when you do this. If it moves, then you move...on to the next boat that is....

Did you know that some engines are intentionally designed with different levels of compression on different cylinders? Because of this, the numbers are meaningless without comparing results to the specifications for the particular engine being examined. I have a V6 Evinrude that calls for a different PSI on the bottom two cylinders as compared to the top 4. Without consulting a service manual, you might start thinking there was something wrong with that engine when there really isn't.


Well I have read about there being a 5 psi difference between the starboard and port cylinders on some Evinrude and Johnson’s, but I have also seen them be exactly the same. Not being a trained technician, I don’t always no what to believe or expect. What motor do you have? I would like to know the purpose of that if that is indeed the case.

I have never read of an intentional difference between right and left. I wonder if that 5 pis was the range of variance allowed or desired from left to right. I've had more than one mechanic tell me the general rule is "be within 10% of stated spec for each cylinder" and it's always good if they are evened out as opposed to being at extreme variance. One of those mechanics is a personal friend and certified Master for both Merc and what was OMC at the time. The Evinrude I have is a 1996 XP 200 crossflow. When he started measuring compression, I got very worried as I recorded the different values as he made measurements as they were quite different at the bottom. He pointed out the way the specs were done in the shop manual, and explained why. I believe it was due to cylinder head temperature. The lower cylinders actually get the cooling water last, so the water is preheated by the time it gets there, especially when running at higher throttle. The lower cylinders get hotter, so the vapor flash and detonation is more significant than the top 4. Dropping the compression spec for those two cylinders lowers them to be better balanced with the other four. I've seen this same thing with horizontally opposed air cooled piston aircraft engines - the back two jugs have different specs.

Re: Buying a used boat ..... [Re: Flippin-Out] #13536747 04/27/20 10:35 PM
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Originally Posted by Flippin-Out
Originally Posted by PlanoPaul
Originally Posted by Flippin-Out
Originally Posted by PlanoPaul
Compression numbers vary depending on the motor, but generally they are anywhere from 100-130psi. And like already said, within 10-15%. I honestly prefer that they are all exactly the same. Also take a good look at lower unit, and see if they will let you check the gear oil for water. As for the boat, check floor and transom. Tilt the motor up a bit and push down on the bottom of the motor, check the transom for flex when you do this. If it moves, then you move...on to the next boat that is....

Did you know that some engines are intentionally designed with different levels of compression on different cylinders? Because of this, the numbers are meaningless without comparing results to the specifications for the particular engine being examined. I have a V6 Evinrude that calls for a different PSI on the bottom two cylinders as compared to the top 4. Without consulting a service manual, you might start thinking there was something wrong with that engine when there really isn't.


Well I have read about there being a 5 psi difference between the starboard and port cylinders on some Evinrude and Johnson’s, but I have also seen them be exactly the same. Not being a trained technician, I don’t always no what to believe or expect. What motor do you have? I would like to know the purpose of that if that is indeed the case.

I have never read of an intentional difference between right and left. I wonder if that 5 pis was the range of variance allowed or desired from left to right. I've had more than one mechanic tell me the general rule is "be within 10% of stated spec for each cylinder" and it's always good if they are evened out as opposed to being at extreme variance. One of those mechanics is a personal friend and certified Master for both Merc and what was OMC at the time. The Evinrude I have is a 1996 XP 200 crossflow. When he started measuring compression, I got very worried as I recorded the different values as he made measurements as they were quite different at the bottom. He pointed out the way the specs were done in the shop manual, and explained why. I believe it was due to cylinder head temperature. The lower cylinders actually get the cooling water last, so the water is preheated by the time it gets there, especially when running at higher throttle. The lower cylinders get hotter, so the vapor flash and detonation is more significant than the top 4. Dropping the compression spec for those two cylinders lowers them to be better balanced with the other four. I've seen this same thing with horizontally opposed air cooled piston aircraft engines - the back two jugs have different specs.

Interesting, i guess that does make sense. I wonder how they achieve the psi difference? Built into the heads, or something to do with the pistons? Is your motor a 96 or 86? I thought they went to loopers in the late 80’s. Again, not a master mechanic here, just asking for my own knowledge. Thanks for the replies.

Re: Buying a used boat ..... [Re: jayhdz78] #13536763 04/27/20 10:48 PM
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Originally Posted by jayhdz78
Guys , wanted to ask when looking into buying a boat especially a little older 95 model , what is the easiest way to do the compression test ? And what should it read ?


You can go to an Oreilly or Autozone and put a 40 buck deposit on a gauge for the compression test. Bring it back for a full refund so it's free unless you keep it or break it.


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