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Correct way to do a compression check? #13332992 11/05/19 04:39 PM
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rkd Offline OP
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Not a mechanic. I am selling boat and would like to do a compression check for the potential buyers. It runs perfectly now but I understand a formal test will make a potential buyer feel more comfortable. 2002 Yamama VMax 150 OX66. I will buy the compression kit this weekend. What is the correct way (most accurate) to do a compression test? 1. Hook up hose and let engine warm up first. 2. Pull kill switch (Is this step to keep gas from being injected?) 3. Remove ALL 6 plugs 4. Screw in test connector and turn starter 5-6 times per connector

Other suggestions/recommendations??

Re: Correct way to do a compression check? [Re: rkd] #13333284 11/05/19 09:40 PM
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Pat Goff Online Content
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You're correct, except pulling the kill switch just kills power to the plugs.


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Re: Correct way to do a compression check? [Re: rkd] #13333487 11/06/19 01:38 AM
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spankyttx Offline
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yup, one more thing, throttle butterflies set to wide open and no choke


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Re: Correct way to do a compression check? [Re: spankyttx] #13333790 11/06/19 02:35 PM
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rkd Offline OP
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Spanky-how do I do that? Remember, no mechanic here.

Re: Correct way to do a compression check? [Re: rkd] #13333808 11/06/19 02:46 PM
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texcajun Offline
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Just guessing, but positioning throttle fully open should take care of this.

Originally Posted by rkd
Spanky-how do I do that? Remember, no mechanic here.


2002 Scandy White, 2002 Yamaha 90
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Re: Correct way to do a compression check? [Re: texcajun] #13333869 11/06/19 03:44 PM
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rkd Offline OP
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May be a dumb question but how do I position the throttle to fully open if I have a hotfoot?

Re: Correct way to do a compression check? [Re: rkd] #13334104 11/06/19 08:59 PM
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wedge it with a stick, a heavy weight like a sandbag, big rock, a barbell weight or any combination of weight, locking pliers (gently, use caution not to mar anything, an old paint stick between the jaws could be considered here), use a kid, your wife or girl friend to hold the pedal at wot (wide open throttle). i'm not familiar that engine but some linkage at the carbs can be wire/zip tied open without touching the the throttle pedal/controls. it wouldn't be a bad idea to make it a two person operation anyway. you could have someone sitting in the drivers seat turning the key and holding the throttle down while the other is at the engine


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Re: Correct way to do a compression check? [Re: rkd] #13334134 11/06/19 09:53 PM
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Some control handles have a lever that is independent of the shifter to raise the rpm. If yours has this then you should be able to go wide open by raising the lever all the way.

Re: Correct way to do a compression check? [Re: Alumacraft 14] #13335823 11/08/19 03:19 PM
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rkd Offline OP
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Thanks everyone for the help. Spanky, the girlfriend-turning key-pushing on pedal routine should do the trick. I am thinking my main concern is getting a consistent (within 10%) reading on each cylinder rather than just trying to get the highest possible reading. If all cylinders are in good shape whoever buys the boat can do a more "professional" check if they want.

Re: Correct way to do a compression check? [Re: rkd] #13335841 11/08/19 03:29 PM
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Cast Offline
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I hate to rain on this, but if I was buying, no test done by the seller would even be considered since it is clearly a conflict of interest.


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Re: Correct way to do a compression check? [Re: Cast] #13336004 11/08/19 05:56 PM
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rkd Offline OP
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Agree with you Cast. Potential buyer should do all of their own checking/testing before buying. I know I will when it's time to buy a boat. BUT, as a buyer, it's nice knowing a basic test was conducted before I hop in the car and drive all the way from say Ft Worth to Lewisville to look at the boat. I guess I could always lie about the results but would look like a total arse when the buyer shows up, does the test, and results are totally off.

Re: Correct way to do a compression check? [Re: rkd] #13336210 11/08/19 10:01 PM
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When I sold a boat last year, I told the buyer I’d do a compression check with him. We got to the 4th cylinder and he says, “ I’d sure like so see some variation in that gauge”! And I responded, “well....I’m hoping NOT too see any”! roflmao

Re: Correct way to do a compression check? [Re: rkd] #13336359 11/09/19 01:38 AM
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If the seller does a compression check in front of me I’d be just fine.


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Re: Correct way to do a compression check? [Re: rkd] #13338865 11/11/19 05:07 PM
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As a former tech, I relied more on a leakdown test. Compression first, since it is easier to complete. Although it may require taking to a shop to perform, it will give you a better idea of overall cylinder condition. A high reading may indicate wear or even a single stuck ring which compression may not catch.

Re: Correct way to do a compression check? [Re: rkd] #13377118 12/19/19 02:34 PM
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Compression test is a diagnostic tool. A Leakdown test is a measure of true powerhead performance.

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