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An Ignorant Question About Checking Compression
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   Fishing Boats, Motors, Trailering, Q & A or Discussion

Posted by WaterChap
10/14/2020 10:12 PM
So I am in the extreme preliminary stages of buying another used boat (probably likely as not I'll keep what I got), and I'm wondering about the whole issue of compression. It seems that checking compression is a little bit harder on an outboard than on a car engine, so maybe I could do it. But as I have been researching I'm starting to wonder why I would bother.

First, if the compression on an engine is bad, it is very likely something non-catastrophic, such as carbon build up. I'm pretty sure my compression would have been pretty dodgy when I first bought my boat, but dozens of cans of Seafoam later, she runs pretty good.

Second, if the compression's bad the engine will run bad. It'll either be radically underpowered or it will idle very rough. If that's the case, why not just run the boat? What is the magic that a compression test gets you?

I'm hoping for something a little more specific than, "well you can do what you want but I always test compression," or "if you don't test compression, you're an idiot." I'm an idiot way before the compression issue and I tend to get skeptical when I can't see the point of a bunch of work. Thoughts anyone?
Posted by fischnrod
10/14/2020 11:02 PM
My experience is that if you have a cylinder with low or 0 compression your looking at a blown engine that needs rebuilt, or you can try to save money and have the one cylinder fixed but I can't see doing that personally. All cylinders should be pretty close to the same compression give or take a little 110 to 120 psi on all the motors I have ever checked but I'm not a boat motor mechanic just my 2 cents.
Posted by WaterChap
10/15/2020 6:08 AM
Thanks fischnrod. Have you ever encountered an engine that had a low compression cylinder but still idled smoothly and ran strong? I’m not looking to set myself up for an engine rebuild, but wondering if it’s common for an engine with bad compression to run well?
Posted by WaterChap
10/15/2020 6:08 AM
Thanks fischnrod. Have you ever encountered an engine that had a low compression cylinder but still idled smoothly and ran strong? I’m not looking to set myself up for an engine rebuild, but wondering if it’s common for an engine with bad compression to run well?
Posted by fischnrod
10/15/2020 8:36 AM
I have not had that experience
Posted by FishingwithRusty
10/15/2020 2:01 PM
checking compression is not hard(the hardest thing is getting to the plugs in some cases, optimax). what motor? i do it cold AND hot as there is variance as cylinder, pistons and rings heat and cool. mercury considers a 10% variance in compression acceptable.

a cylinder can be down/low and the motor idle and even run relatively smooth and unless you know the previous performance of the boat/motor you may not notice the decreased performance of the low cylinder.
Posted by silvertalon
10/15/2020 6:16 PM
15 lbs difference or 15% difference, per bank (starboard or port). A cylinder that is lower than that will usually cause the engine to idle poorly, stall a lot or may be hard to start. It may idle along ok at rpm's higher than 1,000 but not below. You may also notice poor high rpm characteristics. Maybe a couple hundred rpms. All 2 strokes should idle in gear at 650-800 rpms with the motor trimmed level with the waterline. I have seen many motors run ok with over a 15% drop so nothing is a sure bet here. . There are 3 reasons for low compression- sticky or stuck rings, excessive or 'out of round' cylinder wear or bad head gaskets leaking. Carbon fouled spark plugs are a good indication that the rings are coked up with carbon or prone to sticking. A 1/2 can of quicksilver power tune thru the carbs is the 1st treatment to free sticky rings. Spark plugs that look normal but engine exhibits low comp is not good. Pull the cylinder head and inspect and measure the bore. Worn over- spec or 'out of round' require a rebuild. 1st check all cylinders with engine cold. Then test after running up to temp. A low cold cylinder that comes up to spec when warm indicates a wear issue. This is why its important to comp test before buying. If a motor starts easy and idles low in gear is probably ok but I'd test it before buying anyway. It's a simple test . You can buy a comp tester at Harbor Freight for about 15 bucks. It can save you thousands! IMO.
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