Compression test on 30hp 1987 Johnson

CBULL

Cadet
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Apr 13, 2008
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16
I had my boat out this weekend and the motor ran horribly. It took forever to get it started, and once it was started it still ran badly. I gave it a compression test tonight and got 110 psi in the top cylinder and 114 in the bottom cylinder. I'm not sure where to get specs on what this compression should be. Can you please help?

Thanks in advance
 

jbjennings

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Jul 18, 2007
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Re: Compression test on 30hp 1987 Johnson

There aren't any written specs. for johnson outboard compression. From what I've learned here, over 100 lbs. and within 10% of each other is usually o.k.
I'd check spark on both cylinders. Then look to see if you have any leaky fuel lines. Good, new fuel? If none of these, then I'd rebuild the carb. I'd make sure my spark was nice and blue and jumping at least a 1/4 inch gap on both cylinders. For an easy indication, get it running and pull/ground a plug wire and see if it makes any difference in running. If pulling one wire doesn't affect the running, you probably have good spark. If no leaky fuel llines, then clean the carb and you'll probably fix it. I really think it should run o.k. with the compression you had. I know it would run wide open good. Can you be any more specific about "ran badly"?? The details on how it's running are usually very helpful so one of the real experts on here can quickly identify your problem.:)
JBJ
 

samo_ott

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Jun 18, 2006
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5,125
Re: Compression test on 30hp 1987 Johnson

That's good compression. As JBJ said, do you have new fuel and check the spark.
 

tashasdaddy

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Nov 11, 2005
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Re: Compression test on 30hp 1987 Johnson

when was the last time it had been run? has it been sitting for a while?
 

kenmyfam

Supreme Mariner
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Aug 10, 2006
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Re: Compression test on 30hp 1987 Johnson

Maybe in great need of a decarb.
 

CBULL

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Apr 13, 2008
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Re: Compression test on 30hp 1987 Johnson

Thanks for the help!

A few more details about how the motor is running. It's horrible to start; I actually have to prime it to get it started. Once it's running it runs fine- a little rough and runs best a full throttle. I can turn it off for 10 or 15 mins and it will fire right up, but let it sit for any longer and I'll have to prime it to get it going again.
 

jbjennings

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Joined
Jul 18, 2007
Messages
3,903
Re: Compression test on 30hp 1987 Johnson

A good carb soaking and cleaning is all you need, I suspect. If you've never done it before, just be sure you clean out with carb cleaning and compressed air all the tiny passages in the carb and you should be in good shape. Check to be sure your float doesn't have any gas in it by shaking it. don't bend it though.
Good luck,
JBJ
 
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