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  1. #1
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    Please help! 400r down on power but runs great

    2016 Mercury 400r Serial 2B237728

    About 8 months ago, motor was running like a top and out of nowhere snapped the driveshaft.

    Motor was removed from boat while waiting for new lower unit, new lower finally comes in and we mount motor back on. First test run boat is way down on power but runs great. Basically feels like it is down exactly one cylinder.

    Swapped all coils, did cylinder drop test, nothing. Compression is great, fuel octane is proper. Motor runs perfect, but is down several mph (5-7). Also swapped it with the old lower unit (new driveshaft put in) and same story.

    Really at a loss on this one.
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R

  2. Member
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    #2
    Timing was also checked. Motor did bang limiter when driveshaft snapped
    Last edited by linesider 159; 04-19-2023 at 09:56 AM.
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R

  3. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #3
    Try a fresh set of spark plugs.

    If problem persists, check valve timing.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  4. Member
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    #4
    New plugs were done, will check the other. Thanks!
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R

  5. Member
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    #5
    How old is the fuel?

  6. Member
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    #6
    Quote Originally Posted by linesider 159 View Post
    2016 Mercury 400r Serial 2B237728

    About 8 months ago, motor was running like a top and out of nowhere snapped the driveshaft.

    Motor was removed from boat while waiting for new lower unit, new lower finally comes in and we mount motor back on. First test run boat is way down on power but runs great. Basically feels like it is down exactly one cylinder.

    Swapped all coils, did cylinder drop test, nothing. Compression is great, fuel octane is proper. Motor runs perfect, but is down several mph (5-7). Also swapped it with the old lower unit (new driveshaft put in) and same story.

    Really at a loss on this one.
    You said motor off the boat, Throttle setup correctly since its been reinstalled?

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    Also- make sure that it's mounted at the same height.

    What part number gearcase was installed?


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  8. Member
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    #8
    Motor height is same, I have a ton of experience on this hull. It is not only sluggish accelerating but also top end.

    I'll double check part number, my first thought was wrong gear ratio or a tight gearcase, but we put the old one back on and it was same scenario
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R

  9. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #9
    If the fuel pressure is correct, and you've been through everything else, you'll want to check the cam timing. Every once in a while one will spit back hard on shutdown if it hasn't idled a bit, and the chain may jump 1-3 teeth. Rare... but I've seen it happen.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  10. Member
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    #10
    Did a leakdown and found bad numbers, super shock decarbon treatment and power is back! Crazy issue to track
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R

  11. Member
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    #11
    Quote Originally Posted by linesider 159 View Post
    Did a leakdown and found bad numbers, super shock decarbon treatment and power is back! Crazy issue to track
    Good to hear it was something so easy to fix. Take it out and run it like you stole it from time to time!

  12. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #12
    Glad that got you rolling again!

    Strongly recommend Quickare + Quickleen in EVERY tank of fuel for best results.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  13. Member
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    #13
    Whats crazy is it does get run hard time to time. Was was overheated pretty good, ran good the next 3 days until lower blew. Then motor sat for 6 months without being started and was down on power since then. Ran it on portable tank of ringfree after the bad leakdown test and then went and ran the piss out of it. Changed oil and plugs afterwards and she is good to go!

    What sucks is the original shop said they did a leakdown originally, so we sat for 4 months scratching our heads waiting on another shop

    Some of the cylinders were 15-20% leakdown
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R

  14. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #14
    Almost makes me wonder if they weren't doing the leakdown test incorrectly.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  15. Member
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    #15
    I think either they didn't do it properly or didn't do it at all and thought they did.
    2007 Sterling 22xs-Yamaha 250-Dual 10 foot blades-Ipilot-Bobs Jack plate-HDS 8 (SOLD)
    2016 Sterling 220XS: (SOLD)
    2008 SCB Topcat 250 XS (SOLD)
    Current:
    2014 SCB Stingray Sport Merc 300XS
    2018 ES Custom Revolution 25 Merc 400R