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Compression Range Question

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6tring

Contributing Member
I am going to take a look at a1970 Evinrude 9.5 hp for possible purchase.
The motor is non running. Owner claims it will start than sputter than die out. Ok, so this sounds to be fuel issue? I told present owner that I would like to take a compression test. He is fine with that. By way of pics I seen of it for now it looks clean under the hood. Fresh water motor. My question is what would be a minimum compression amount that it should have before I would go any further. Motor is fairly priced for non running.

Thanks for any input on the matter. Don
 
You probably won't see that on a 9.5. I'm pretty happy with 60 to 70 psi. I've had my gauge quite a while .....40 years, it is pretty trustworthy, maybe reads a tad low. The 9.5's often have cracked coils. Carbs are simple but can plug up at the end of the low speed needle where the tiny chamber spits fuel into the carb throat, causing a "cough" at low speed. If you get a decent one, they are fantastic fishing motors. I have repaired many. Just sold a longshaft 1968 for 650 last summer. They are a big bore with shorter stroke and won't quite keep up with the older 10hp's. But I love 'em.
 
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As long as both cylinders are about the same, within 5#
The results vary with the gauge or the person pulling the rope.
Cheapo Harbor freight tester will give CHEAPO results.

Sold a 225 Mercury. The comp was 160# all 6 cyl.
The buyer said he wanted to do a comp test.
He pulls out a NEW Harbor Freight tester.
His results were 85# 90# 110#
Then I stopped him and gave him my MAC TOOL tester.
The results went back to 160# on all cylinders.
That show how crappy the HF stuff is.
 
I use the smaller Merc tester, it has no hose to allow for expansion. Also, both testers and all torque wrenches are calibrated to MIL specs by some buddies in local Guard every couple of years.

9129287.jpg
 
Excellent information, guys. You are so correct Jerry. Pappy, I'm sure your gauge is top quality as well. I regularly test my gauge with the USG unit on my 48 year old air compressor. I also cross check with my Devilbiss air purifier/regulator that I use when I paint. My gauge does have a flexible hose about 12" long so I can use it for a variety of applications. I can even grab the lowest cylinder on the Merc Tower of Powers. Thanks guys.
 
I purchased the motor. Good compression at 70#+ each cylinder. The rubber cup on the end of the kill switch was rotted along with its lead wires were frayed. Other than that everything else looks solid. Lower unit was clean, no water mark or corrosion. Will be my spring project. I removed the plugs and gave each cylinder a good shot of stabill fogging spray for the winter rest. I plan to go through the everything, ignition system, uel cooling system along with replacing the head gasket to have a dependable motor in the end. I also ordered a 1970 Evinrude shop manual.
 
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Sounds good. Get yourself a new thermostat too. More advice for you if you only ask. Got some sharp techs here.
 
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