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  1. #1

    Default Mariner 25hp won't start

    Here's a strange one. We bought an older boat & outboard recently. On it's second run on the water, the Mariner 25hp stopped mid-journey after running fairly well (but maybe a bit low in power, I think). Just gradually slowed down, then stopped altogether.

    We got rescued & towed to shore, thankfully, but now at home the engine won't start at all.

    I have checked the fuel situation (replaced hoses & tank with new ones) and changed the plugs with new ones after setting the gap to 70mm.
    If I take the top plug out, earth it to the crankcase & turn the engine over (electronic ignition), I see a spark & the engine will actually start & runs for a short while (without the plug in it!), then stops.
    If I replace the plug, no start at all.
    If I take out the lower plug & do the same, I see a spark, the engine coughs but doesn't want to run. Both plugs back in & she's dead again.
    Any ideas anyone, please?

  2. #2
    Join Date
    Sep 2003
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    New Tripoli, PA, USA!
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    Default Re: Mariner 25hp won't start

    Someone want to take this one on?

    I'm baffled!

    Jeff

  3. #3
    Join Date
    Sep 2010
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    21,182

    Default Re: Mariner 25hp won't start

    Proper information MUST be posted---Post compression numbers too.------It says in post #1 that plug gap is 70mm , somehow I say that is not possible ??----Checked the flywheel key on this motor ?

  4. #4

    Default Re: Mariner 25hp won't start

    Quote Originally Posted by racerone View Post
    Proper information MUST be posted---Post compression numbers too.------It says in post #1 that plug gap is 70mm , somehow I say that is not possible ??----Checked the flywheel key on this motor ?
    Apologies, misplaced a decimal point - plug gap is obviously 0.70mm (0.28"). Haven't had a chance to test the compression on the engine (don't yet have a tester & was hoping to find there was some sort of basic remedy before investing in one).

    I'm not by any means a qualified mechanic, so I don't actually know what I'd be looking for in 'checking the flywheel key'. What information would that tell me & what would I then need to do with it, please?

    The engine was running ok when we bought the boat, as far as I could tell. It started with no problems at all & ticked over nicely when we were buying.

    It then started & ran ok on our first river trip, so for the second outing I thought it would be ok to take the boat out on smooth water just offshore. The engine seemed fine at first, a little disappointing as to thrust, but we had 4 people on, so I just thought maybe I had overestimated the ability of the Mariner, putting this down to my inexperience as a boatie. After about 30-40 mins however, the engine began to run more & more slowly, then petered out altogether.

    It would not restart, even though we had plenty of fuel.

    There was a fairly fast outgoing tide, but fortunately we managed to paddle to a nearby island. We were still within cell range, thankfully, so we called the local volunteer rescue guys, who towed us back to the boat ramp.

    Since then, as I stated originally, I have had no luck in starting the Mariner at all. The battery is well up, there seems to be a spark on each of the new plugs, I have replaced the fuel tank & hose with new ones, symptoms remain as in my original post. If we keep spinning the flywheel with the ignition the engine will sputter, but not run, no matter whether I use choke or not.

    Sorry I'm such a novice in all this, but I guess if I wasn't, I wouldn't be coming to a forum...

  5. #5
    Join Date
    Sep 2010
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    21,182

    Default Re: Mariner 25hp won't start

    Serial number / year of the motor?----- Did you install a new water pump impeller ?-----A golden rule for a " new to you motor " I say.-----If a 2 stroke are you mixing gas and oil at a 50:1 ratio ?

  6. #6
    Join Date
    Jun 2013
    Location
    Duluth Mn
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    4,072

    Default Re: Mariner 25hp won't start

    Sounds like an overheat or lack of oil. Racer suggests compression test....agreed. If that checks out....like 120 psi, then a spark gap test. The spark should be able to jump nearly 1/2" with a sharp snapping crackle. Have both plugs out and create a gap tester, even as simple as a #2 Phillips screwdriver pushed into the spark plug boot and held about one half inch from a bolt or other metal ground on the powerhead.

  7. #7

    Default Re: Mariner 25hp won't start

    Serial number on the silver plate says 'Mariner 695000339'. Not sure of year, probably 80's or 90's by the look of it. As I say, the motor fired up & ran ok on the previous journey & on the cleanout at home after - the water seemed to be flowing well, but that could have changed during the second journey, I suppose. I'll check the impeller. I hope I got the fuel mix right, but 50:1 is what I put in the tank.
    I'll buy a compression tester. The sparks seem to jump ok too...

  8. #8
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    Jun 2013
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    Default Re: Mariner 25hp won't start

    The first sign of overheat on these is slight melting around the coils.....take a look.

  9. #9

    Default Re: Mariner 25hp won't start

    Yes, I did notice some distortion in the plastic parts around the coils... I guess if it's overheated, then the engine's a goner?

  10. #10
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    Jun 2013
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    Duluth Mn
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    Default Re: Mariner 25hp won't start

    Maybe so. A little lite oil and compression test is next.

  11. #11
    Join Date
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    Default Re: Mariner 25hp won't start

    First thing on these old outboards is a new good quality water pump impeller, as Racer suggests. These Merc/Mariners are a little bit touchy to get the supply tube into the pump discharge fitting... Maybe because it's so black in there.....gotta be careful. Have your beer AFTER the job is done.

  12. #12

    Default Re: Mariner 25hp won't start

    Quote Originally Posted by timguy View Post
    First thing on these old outboards is a new good quality water pump impeller, as Racer suggests. These Merc/Mariners are a little bit touchy to get the supply tube into the pump discharge fitting... Maybe because it's so black in there.....gotta be careful. Have your beer AFTER the job is done.
    Thanks everyone for the comments. A friend put me onto an outboard mechanic who did a compression test. Top cylinder 50psi, bottom 100psi. Probable repair more costly than the value of the motor. Yup, should have done the impeller before running the engine. However, the lack of power I was experiencing probably indicated the compression was down anyway. If Id been more experienced, or had help, I would have known to do the compression test BEFORE buying, not just see the engine running.

  13. #13
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    Default Re: Mariner 25hp won't start

    Don't fret, brother.....just ask the forum here....thousands of years experience is only a few clicks away. If I had this when I was younger, I might have been a decent mechanic by now.

  14. #14
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    Default Re: Mariner 25hp won't start

    Don't just throw it away--lots of valuable spare parts.

    Jeff

  15. #15
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    Default Re: Mariner 25hp won't start

    You might do well to take it apart and start listing stuff on eBay. Sign me up for the switch box.....they are really a pretty high failure item. You can pick up some extra cash, like I do. Then I can spend more time fishing with wife and son.
    This screenshot is just an example of value new.....I buy almost all my CDI stuff right here @ Marineengine.com. Why? This organization has helped so many people save money and aggravation, through the expertise of its members, that marine repair has become enjoyable to many. Why not learn and benefit from the knowledge and experience of others? And all for free.
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  16. #16
    Join Date
    May 2015
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    2,452

    Default Re: Mariner 25hp won't start

    That switch box won't fit the OPs motor. The OP has a Yamaha built Mariner.

  17. #17
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    Default Re: Mariner 25hp won't start

    Oops! Yes, I think the Yamaha version is cheaper.....I never checked the serial number. Parts are valuable for sure....especially the lower unit.

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