85' Merc 90hp I6 misses under load

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susqray

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My '85 90 hp I6 runns ok for about 5 or 10 min, then develops a nasty miss.<br />The Data: 16' Larson runabout (~2000#), 19P aluminum prop, 50:1 gas, WOT RPM cold - 4100 ( a little low), speed cold - 29mph(gps). After 5-10 min, WOT PRM -2000, speed -10mph (gps). Checked fuel lines, checked vent, rebuilt pump, cleaned carbs, reset timing. Idles fine when hot, only misses under load.<br />Tried pumping bulb, portable tank, no help.<br />pulled the plugs when I got home, and 6 was clean (too clean). Pic of plugs here:web page cylinder one at top, six at bottom.<br />Is it likely I have a bad coil? Plug?<br />I did find some corrossion on the ground wire for the #6 coil and cleaned that. Haven't tested since (it's a 45 min drive to the water.)<br />At idle in forward, I did hook a timing light to each plug wire and it fired, but couldn't (didn't) test under load.<br />Thank you
 
eurolarva

eurolarva

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Re: 85' Merc 90hp I6 misses under load

YOu should do a compression test. Looks like you are getting water in number 6 cylinder. Probably a blown head gasket
 
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susqray

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Re: 85' Merc 90hp I6 misses under load

Compression (cold):1-152<br /> 2-148<br /> 3-150<br /> 4-145<br /> 5-150<br /> 6-145<br />I do get some water spray out of the exhaust ports on the leg. is this related?<br />also notice tach needle drops when I adjust trim/tilt.<br /><br />Thanks
 
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Chris1956

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Re: 85' Merc 90hp I6 misses under load

Susgray, I also think you are getting water in #6. You may also be getting water in #2. The plugs from my inline six usually have the most oil and carbon on the number 2,4 and 6, since they run richer than the others due (I guess) to gravity. I also do not like the look of plug number one. It should be cleaner than plug number two. That's why I suspect water in #2. If it doesn't get better when you clean up the #6 coil, you might remove the exhaust cover (Port side of motor) and inspect the inner jacket and plate for holes or cracks.
 
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susqray

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Re: 85' Merc 90hp I6 misses under load

That came to mind. Also probably will change stator. Just checked it and it tests about 200 Ohms high. Plugs may have been switch around previously I'll put new ones in the next time I test, just in case.<br /><br />Thanks for the ideas.
 
eurolarva

eurolarva

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Re: 85' Merc 90hp I6 misses under load

If the corrosion is bad enough it will create too much contact resistance and you might not be getting a spark. I would take one of the other plugs that has the most carbon on it and put it in the 6 th cylinder. Run motor for awhile and then re examine the plug. If the tip is now clean you are getting water in that cylinder and it is steam cleaning the plug. Hopefully your cleaning the wire is all that was needed. Your compression numbers look really good so it would be hard to believe you are getting water in cylinder
 
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PaulRuffo

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Re: 85' Merc 90hp I6 misses under load

Hello,<br />I also have a 1985 Mercury 90hp outboard engine with a very similar problem. When I first start the engine and begin to drive forward, the engine has a ton of power, but within 30 seconds, it bogs down to the point where it wont rev above 2,300 rpms while in gear. Interestingly, I was able to very slowly ease into the throttle and get the engine to catch. The rpms climbed to 3,900 and we ran like that for 5 minutes or so. Once we tried to increase the throttle, the engine bogged and we were unable to make it rev up in gear again.<br /><br />I would be very interested in hearing about what your problem was when you figure it out as it may help me as well.<br /><br />Thank You,<br />Paul Ruffo
 
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susqray

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Re: 85' Merc 90hp I6 misses under load

OK, I finally got it out on the water again, and I'm pretty sure I'm getting wated in #6. Piston is shiny. Used new plugs (Autolite instead of champion.) I de-carbed the engine, and i get 2500-3000 rpm WOT under load.<br /> Can the water be coming in the exhaust side or the lower crank seal? Is this the likely source?<br /> If I take the powerhead off, can I replace the lower seal without tearing apart the whole engine?<br /> Which parts could be leaking on the exhaust side?<br /> The other cylinders *seemed* to be good.<br /><br />Thanks for the help so far, SusQray
 
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susqray

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Re: 85' Merc 90hp I6 misses under load

Can i remove the port side cover without removing the powerhead? How do I get to the lower bolts?<br /><br />Sorry for double posting
 
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phatmanmike

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Re: 85' Merc 90hp I6 misses under load

just to rebuke what eurolava said...
Your compression numbers look really good so it would be hard to believe you are getting water in cylinder
Click to expand...
thats not true, i have a motor right now that has 160# on each cylinder, and it was getting water in from the esxhaust cover, my lower plug was brand new looking, and my top plug lookled old, swpped plugs and same thing, so you cant use that thinking....<br /><br />m ikey
 
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Chris1956

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Re: 85' Merc 90hp I6 misses under load

Susgrey, You can remove the port side exhaust cover without pulling the powerhead, however, it is so much more convenient to work on the the powerhead (on your bench), when it is removed. Also, it is possible (saltwater use => very likely)that one or more of the inner water jacket cover bolts could break, when you attempt to remove them. Also it is tricky to replace the inner water jacket and gasket, while the powerhead is still installed.<br /><br />Also water is likely entering cylinder through a hole in the inner water jacket cover or baffle. If it is the baffle, that is as good as it gets.
 
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susqray

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Re: 85' Merc 90hp I6 misses under load

Thank you one and all!<br /> Pulled power head. Inside exhausr baffle was clean at 6 and showed some steaming at 4 and 2. Baffle looked good. I think the gasket was leaking at the water passage or the lower bearing seals were bad.<br /> I replaced the exhaust baffle gaskets (inner and outer), the lower crank seals and o-ring, the cylinder cover gasket (while I had it off the boat) and the base gasket.<br /> I had it out this morning and the miss was gone. Still low on rpm's(about 1000 low), but when I got home I found the timing was retarded about 4-6 deg at full advance. Hopefully that was the problem there. Throttle appears to be opening all the way. Ran consistent @ 3700.<br /><br /> Once again, thank you for your help so far.<br /><br />Susqray
 
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