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  1. #1

    Post 125 mercury 2 stroke

    So I've owned this boat/motor for 3 years, have had a minor issue going from idle to full throttle, it has a 3-4 second hesitation and then it jumps up and goes like it should. Idles fine and planes out beautiful, runs like a rocket. I decided this year to have it looked at before high school tourneys start back up in August. After telling mechanic what my issue was he asked about the carbs and last time they were rebuilt, I've owned it 3 years, unknown when they have been rebuilt if ever. He said more than likely the jets are stopped up. So I bout 4 kits and he rebuilt the 4 carbs.

    I took it for a spin and it was the same or even worse during the hesitation period. Again idle and plane were great, super fast for a 125 motor. He took it all apart again and said he thinks the #4 carb needs to be replaced. So I ordered a carb, but before it came in he called me again and said that wasn't it he thinks the 4th cyl piston is scorched. He said he did a compression test on all 4, got 125 on 1,2 & 3, only 95 on 4 so he said the piston and rings were possibly bad and needed a new powerhead but he didn't do that kind of engine work. The guy he recommended that does it is 9-12 months out on work.

    Changed plugs and fuel filter first of this year. Fresh gas in tank with ethanol treatment.

    I've done a little reading and someone mentioned the fuel pump. So tonight I did the removal and manual fuel pump test and from what I can tell the diaphram is ok because it didn't leak on me when pressurized.

    I have a buddy that just did a powerhead replacement last year and he's offered to assist me. But I'm wanting to make sure I am headed in the right direction before spending unnecessary money that isn't the solution.

    Any thoughts, ideas or tips are appreciated.

  2. #2
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    5,413

    Default Re: 125 mercury 2 stroke

    First...get away from everything this so-called mechanic is telling/selling you. You need to test compression yourself. You have the 2+2 motor and they are known for this bogg if carbs/linkage/accelerator pump/injectors are not setup properly or dirty...
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  3. #3
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    A compression test is the FIRST step to take in trouble shooting.----Remove exhaust cover to look at the #4 piston.-----And I agree with faztbullet , perhaps find another mechanic.

  4. #4

    Default Re: 125 mercury 2 stroke

    Yeah he was recommended from a friend, I have since told my friend to not recommend him to anyone again. I'm looking for another shop to give me a second opinion to get to the bottom of it.

  5. #5
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    Some shops will tell folks what they want to hear.----I know my truck mechanic does not tell me any wild tales.----He knows what I know.----Pull the exhaust covers only 25 bolts or so.----That will tell you the story.----And before you ask there is no head gasket to cause low compression on that motor !

  6. #6

    Default Re: 125 mercury 2 stroke

    So I pulled the exhaust cover off a few months ago just looking to see and learn the motor parts. When that's pulled off allci see is the carbs and choke valves. How would that tell me when i need to know? Just curious.

  7. #7
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    You need to look up what the exhaust cover looks like!!------It is a large cover on port side of the block.------There are no choke valves on this motor either.------Sorry to be blunt here.-----Time for some reading / research on your part.

  8. #8

    Default Re: 125 mercury 2 stroke

    I was thinking the cover on the front that covers the carbs, I see what you are referring to on the exhaust cover now, I'll check it later this weekend. All 4 carbs have chokes on them, I did call it a valve but that was incorrect, just choke. Thanks, trying to learn as much as I can, I am a car guy but boats are a different breed.

  9. #9
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    I doubt they have CHOKES.-----Throttle plate in each carburetor.----No choke plates.

  10. #10

    Default Re: 125 mercury 2 stroke

    Pulled the exhaust cover tonight. See pic. Other than a little black no metal shavings or pieces. Is this nornal looking or too much black?

  11. #11

    Default Re: 125 mercury 2 stroke

    Hoping the pix from my phone uploaded.

  12. #12
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    Rotate the flywheel and see what #4 piston and rings look like.

  13. #13

    Default Re: 125 mercury 2 stroke

    Rotated it a few times and in my opinion it looks just like the other 3. Nothing looks out of the ordinary.

  14. #14
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    But you say you have low compression on #4 cylinder.-----So something is wrong with piston , rings or cylinder wall on #4 I say.

  15. #15

    Default Re: 125 mercury 2 stroke

    Compression is 95 compared to 125 on the others. Mercury doesnt consider low compression unless it's 90 or below, so yes it's close to that number and lower than the others.

  16. #16
    Join Date
    Sep 2010
    Posts
    24,015

    Default Re: 125 mercury 2 stroke

    Some will argue.-----There is something wrong with #4 cylinder.

  17. #17
    Join Date
    May 2015
    Posts
    2,807

    Default Re: 125 mercury 2 stroke

    Quote Originally Posted by bowhunterokie View Post
    Compression is 95 compared to 125 on the others. Mercury doesnt consider low compression unless it's 90 or below, so yes it's close to that number and lower than the others.
    Mercury considers anything under 120 a problem I believe. Get a different tester with a 90 degree fitting and try it again.

  18. Default Re: 125 mercury 2 stroke

    This is something you can do yourself. Go to an auto parts store and buy a compression tester. They will be a lot less expensive than a mechanics time. Remove your spark plugs and screw in the tester in one of the plug holes. Turn your motor over a few times until tester gauge tops out. Write down reading. Repeat on all cylinders then compare results. Better yet post results and these guys can get you pointed in the right direction.
    Quote Originally Posted by bowhunterokie View Post
    Yeah he was recommended from a friend, I have since told my friend to not recommend him to anyone again. I'm looking for another shop to give me a second opinion to get to the bottom of it.

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