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  1. #1
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    My sickly Mariner 75

    I bought a 4 cylinder (1984 ish) Mariner 75hp outboard in the winter. It's just come back from a service with my local Marine Mechanic and I've been told to "get rid of it"
    Basically it starts OK and if you can get it WOT it flies. Problem is as you open the thottle it can die. It's really difficult to re-start when this happens. Youv'e got to open the thottle at just the right rate, it's impossible to cruise at slow speed. I'm told the crank and seals are worn and it's not worth repairing. I've no real reason to disbelieve the mechanic, he should know better than me but if he's wrong I may be scrapping a perfectly OK engine that needs a $100 fix. I suppose I just don't want to believe him! Anyone seen this engine problem before? He says it's common on these engines.

  2. #2
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    It is a common enough problem that is usually caused by a bottom main seal failure resulting in excessive play in the lower crank bearing,causing the labyrinth seals in the lower reed block to rear and weaken the lower two cylinders at low speed.A very white porcelain on the bottom sparkplug is another sign(steam cleaned).However if this is your problem the motor should be quite noisy mechanically ,very noisy.If it's not your trouble could be a simple(for someone who knows this motor) sync and link and carb adjustment away from running great.Realize also the choke won't work with the throttle open at all,choke with throttle closed,once firing you can advance the coldstart lever,while still choked.To start try opening the idle ari screws 1/3 turn,Chris

  3. #3
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    Thanks for that Chris. The engine isn't especially noisy. I'll check the bottom plug against the top three. I have been trying to start with throttle and choke - now I know better!
    I've also noticed that the engine won't start cold unless I've pushed so much fuel into the carbs that it's spilling out into the water, I've also seen this on my old 50hp merc but surely it's not right?

    I'm in the Uk so I guess you are up very early or a lousy sleeper?

    Phil

  4. #4
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    It seems the big 4 cylinder inlines like yours run well or run poorly. The reed blocks can wear beyond a usable condition with a bearing that is loose and not to noisy. If your motor is an '84ish it has an enrichiner system and not a choke. when starting cold try just actuating the "choke" button for a count of 3 or 4 without spinning the motor. Then crank the motor without any "choke". Some motors like a longer "count" some less, some like a touch of throttle, others don't. Worn reed blocks, water leaks, linkage or perhaps just a tired old motor all will make 'em run poorly. Sometimes a can of Power Tune, Seafoam or other engine decarbonizer can help if it's tired, but it won't do anything for the mechanical issues. It seems your mechanic would have looked at all the basics and this isn't the first one of these he's seen.
    Bruster's Mercury solid motor mounts, details here:
    http://www.powercatboat.com/Bruster/Bruster.html
    I'm been living in fast forward, now I need to rewind real slow....

  5. #5
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    Thanks,
    I'll try the advice on starting. Just checked the plugs and to be honest they've only done 30 minutes running from new but I was suprised just how white they all are - like brand new, they are all identical.

  6. #6
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    I had the same problem on one of my older 85's. finally tore it down and found it had a broken reed so it would be hard to start but would run fine at full throttle if you could get it there.

  7. #7
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    Just started it from cold, 4 seconds of "enricher", turned the key and starts straight away. Thats one problem sorted. Wonder if it'll start so easy after it's stalled when I open the throttle on load.

  8. #8
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    If you suspect a broken reed hold a business card 1/2" in front of the carb throat. If you have a broken reed it will spit gas back on the card.

    Quote Originally Posted by RONNIE View Post
    I had the same problem on one of my older 85's. finally tore it down and found it had a broken reed so it would be hard to start but would run fine at full throttle if you could get it there.
    Bruster's Mercury solid motor mounts, details here:
    http://www.powercatboat.com/Bruster/Bruster.html
    I'm been living in fast forward, now I need to rewind real slow....

  9. #9
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    Can I do this test on the muffs or do I need to be accelerating on load? - Many thanks for the help so far.

  10. #10
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    Anywhere, it just needs to be at idle. If it starts quickly it's not very likely a reed is broken.

    Quote Originally Posted by Phil Abbott View Post
    Can I do this test on the muffs or do I need to be accelerating on load? - Many thanks for the help so far.
    Bruster's Mercury solid motor mounts, details here:
    http://www.powercatboat.com/Bruster/Bruster.html
    I'm been living in fast forward, now I need to rewind real slow....

  11. #11
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    Checked the business card in front of the carb throats, no fuel spitting back. I'll put it back on the water and check the plugs after some running. Then I'll make a decision on what to do with it. Many thanks for all your help. Phil

  12. #12
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    Quote Originally Posted by Bruster View Post
    Anywhere, it just needs to be at idle. If it starts quickly it's not very likely a reed is broken.
    Hi, One last thing if that's OK, Is this motor likely to start OK when warm without "enrichner" or do they usually need some to start even when warm (normal operating temp)? I'm thinking it's not worth putting in the water again unless I know I'm going to be able to start it more easily when it stalls. I can live with it stalling sometimes as long as I can start it again afterwards.

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