60 hp Mercury running rough

Jlat

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Joined
Jul 30, 2015
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Hello, I'm new to this site, sorry if I've posted this in the wrong area. I have a 1986 Mercury 60 hp outboard two stroke, began having problems with it the last few trips out, put the boat in the water, motor fires right up, idles great, run it for 5 minutes or so then all of a sudden loses power drastically as if there a dead cylinder or something. The boat will only reach about half the speed it normally would and motor sounds like it's struggling and will not idle anymore. Replaced spark plugs because they became fouled with oil, compression tested all cylinders have around 100psi
Put the boat back in the water runs great again for about 5-10 minutes and same issue arises? I'd like to narrow it down before I start replacing everything, any help would be much appreciated!
 
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