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Thread: 350 running weak after altitude drop

  1. #1
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    350 running weak after altitude drop

    Hey everyone I wanted to see if anyone else has had this same experience as I am having and if there is anything I can do to fix it.
    I recently took my boat up to around 6000 ft above sea level to cruise around. I ran it for 3 days without any problems except the expected sloppy idling and 20 percent or so loss in power. Well since I have brought the boat back to where I live which is sea level, I have noticed a definite difference in exhaust/engine sound. It sounds a bit more muffled and less "crackly" if you will then it used to. I also think Ive noticed a slight power loss.

    Does anyone know if there is some sort of altitude sensor on my 87 350 mag that may have gotten stuck when i was 6000 feet? Sounds a bit technical for such an old engine.

    Another observation, I am noticing a large amount of exhaust residue on my transom now. I had it quite a bit at altitude where it would accumulate at the water line and above and I have never had this before. Ever since, I still have quite a bit of this black residue now on my transom, does this have something to do with it or am I imagining things?

    thank you all!!

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    When you ran your boat at a very high altitude you were no doubt running an unusually very rich fuel/air mixture there with that very thin air at 6000 feet which created the soot out your exhaust tailpipes and on your transom.

    You may have also carboned-up your engine's combustion chambers, piston tops, & sparkplugs.

    The very first thing I would do is change-out your sparkplugs~~or at the very least clean them to remove that carbon build-up.
    A product called Valve -Tech sold at marine & auto parts stores will also aid in removing the engine carbon build-up if used per spray can directions.
    I have also used a fine mist spray bottle to spray a fine mist of water into the air intake at a fast idle to aid in combustion chamber, valve, and piston top, carbon removal.

    Disclaimer~
    Caution: Be sure not to spray very large amounts of water in a running engine or serious engine damage may & will result.
    Last edited by silverghost; 09-24-2012 at 11:21 PM.
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    did you buy fuel up there?
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    Ok guys I will start with the spark plugs then. thanks for the insight as I am by no means a mechanic.
    -Buizilla, I have been running 91 octane lately as that is what is sold at my home marina. I purchased two tanks of fuel up there and they only had 87. I have since ran through the stuff and am now running 91 again. what are your thoughts on the fuel from up there?

    much appreciated folks, will post results when I clean/swap out plugs. Any suggestion on special spark plugs while I'm at it?

    -Patrick

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    MEFI sensors

    Mercury Fuel Injection uses a MAPS sensor in the intake manifold to determine Manifold Air Pressure - this would record different manifold pressure at altitude than at seal level. Some of these sensors like the IAC - Idle Air Control can get fouled with fuel residue and stick in one position or another. This usually requires a session with carb cleaner.
    Dr. d

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    [ QUOTE=$originalposter]{$pagetext}[/QUOTE]

    Dr., Is this true even for the older, carb. versions of the 350 mag like mine?

    thanks,

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    Dr, a '87 350 Mag has a carb... no IAC or MAP
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    Hye folks reporting back......mrfixx and silverghost were correct, I replaced the spark plugs and sure enough they were carbon fouled to hell. Popped the new ones in and she sounded like her old self again. STILL have aproblem though!

    ran her for about an hour or so around the river yesterday and I noticed several things still:
    -exhaust smelled gasoline-y as well as inside of the pipes after I took her home.
    -Black soot on transom still
    -could be my imagination, but I swear I was getting crappy fuel efficiency as I could practiacally watch my gauge dropping.
    -Once again could be my imagination but slight drop in performance toward end of day when I should have picked up more speed with just myself and 1/8 of a tank of gas.


    Once again I am not even close to being a mechanic, but this may all tell me that there is way too much fuel getting poured into my engine right? If this is the case what could be causing this?? I also hope that I dont have to change my plugs agian as it was a total pain in the a$$.

    that's the last time I take my boat to tahoe (6,000 ft alt)

    Thanks everyone!

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    You were running way rich at altitude. Not only were your plugs fouled but everything else is sooted up as well (combustion chamber, exhaust, etc). It will probably blow out with time. We always had to re-jet and lean out carbureted snowmobiles when we ran them at altitude. Aircraft typically lean out at 3000 feet.
    Patrick

    1968 Donzi Ski Sporter C16-512

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    [ QUOTE=$originalposter]{$pagetext}[/QUOTE]

    Ok, but does it sound like for some reason I am STILL running too rich? Now that I am at lower altitude it should've gone back to normal right?

    -Patrick

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    Not that this is my forte , because it isn't , but aren't the stock Merc engines supposed to be quite happy with a diet of 87octane ..anything more is overkill ? ?
    Just because something's old doesn't mean you throw it away !

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    make sure you are firing on all cyl, when you changed the plugs you might not have gotten all the wires back on snuggly. also check your choke that it is working and opening up. might be a good time to read the plugs and see what they look like. keep an eye on your oil has well fuel could wind up in the oil if it is running real rich
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    What brand of carburetor do you have on this engine ?
    If it is a Holley it might be a ruptured power valve diaphram that is causing excess fuel to dump into the engine's carb & intake.

    I would run it for a bit at high speed to try to blow-out the old carbon inside the combustion chambers, on the piston tops, & on valves.

    Also~
    Check for gasoline in your motor's lube oil as Matty suggested.
    Last edited by silverghost; 09-27-2012 at 01:51 PM.
    "BENCHSEAT 18" ~~YellowJacket~~ project owner~
    1929 Chris~Craft 28' Tripple Cockpit Mahogany Speedboat / A-120-A 845 Cu.In. 375 H.P. Chris~Craft V-8 racing engine.
    24' American Skier
    Super Eagle 454 HO Skiboat
    1991 454 SS Chevy Super Sport Pick-up for towing my "Toys".

    There is no such thing as going too fast ~ ~~
    OR~ Being too old~ for a new "Toy"!

    Brad Hunter
    Huntingdon Valley Pa (Just outside Philly)~
    Ocean City NJ
    silverghost1926@msn.com
    215 947 4676 (PA Home)

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    Ok, but does it sound like for some reason I am STILL running too rich? Now that I am at lower altitude it should've gone back to normal right?

    -Patrick


    Everything should be normal back at lower altitude. You might want to try running some SeaFoam in the tank. I would think a few hard runs would do a pretty good job of cleaning it out. Here's a pretty good article on carbon deposit removal.

    http://www.hemmings.com/mus/stories/...feature19.html
    Patrick

    1968 Donzi Ski Sporter C16-512

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    Thanks guys I will try all of the above and report back.

    -Matty, Ran about 58 mph turning 4800 RPM yesterday for quite some time, thos enumebrs would be substantially less if is wasnt firing on all 8 I would think yeah? Ran hard for a while maybe the soot on the transom is all the carbon being blown out and I am imagining the lsos of performance and bad fuel economy? What are your thoughts ont this?

    -Silverghost, I just have the regular stock one that the engine came with...

    -Sandog, nice article very informative, really sheds some light on the internal happenings of htis carbon deal thank you.


    Any ideas about maybe I am jsut blowing off all that built up carbon still? I probably have only 2 hrs on it since ive been back from altitude.....But would the exhaust pipes have a gasoline-y odor if it was simply carbon being blown off?

    thanks all

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