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  1. #1
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    Mercury 2.5 Exhaust leak

    Hi guys!
    Id appreciate some help/guidance with a 1995 Mercury 200 2.5l carb motor I bought a couple months back for my flats boat.
    The engine runs pretty well with good compression but it seems to be letting exhaust out around the upper solid engine mounts and onto the steering area.
    It is especially noticeable when the outboard is in the water and the bobs machine exhaust housing is under water. Instead of forcing exhaust out the exhaust side it gets forced out the lower cowling in the area mentioned above.
    I have been having trouble getting the motor to idle while on the water and im pretty sure its a back pressure problem from this leak because it will idle perfectly on muffs with no restrictions. Above idle the engine runs beautifully on the water.
    I understand the powerhead will most likely need to come off for repair.
    Any advise appreciated.
    Tony

  2. #2
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    Quite a few posts here about solid mounts that aren't fitted / installed properly causing that sort of leak.
    Let's go Brandon! .

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  4. #3
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    Welcome to scream and fly! I had to do a little cleanup on my mounts and cases when I installed bobs solid mounts. I also used a little Permatex grey on my gaskets.
    1990 Shadow bass boat w/ 2.4 200 Merc. Totally resto'd boat and love it!

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  6. #4
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    Thank you for your reply!
    Other than the powerhead base gasket, what other gaskets need to be replaced to address the upper mounts?

    I have the weekend free so I may attempt to tackle this job if I can get my hands on all the parts!

  7. #5
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    Dig Around on this form and you will probably find a link to a manual for your motor in PDF. I think there are about three gaskets or so that you will need. Has been a year or so since I did mine and a don’t remember the details. If your motor has been in salt water, it might take a little work to get it apart. I would plan for it being torn down more than the weekend if you have the ability. Good luck. It is not hard, just takes time. You will definitely need a cherry picker or hoist to lift the powerhead. The best way is to use the tool that screws into the flywheel and has the eye for lifting. Otherwise, make a sling for it.
    1990 Shadow bass boat w/ 2.4 200 Merc. Totally resto'd boat and love it!

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  9. #6
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    If you work on these ole motors often , u should b able pull power head , swap gaskets , add little gasket sealer of ur choice around the mounts and have it back together in couple hours, you don't have to go crazy and strip a bunch of parts off

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  11. #7
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    Quote Originally Posted by Merc 2.5 View Post
    If you work on these ole motors often , u should b able pull power head , swap gaskets , add little gasket sealer of ur choice around the mounts and have it back together in couple hours, you don't have to go crazy and strip a bunch of parts off
    If he runs in Miami salt water - good luck getting those power heads studs loose in a few hours without much damage to put everything back on lol

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  13. #8
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    That is exactly why I suggested that his junk might be torn down for more than a weekend ...salt water
    1990 Shadow bass boat w/ 2.4 200 Merc. Totally resto'd boat and love it!

  14. #9
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    Quote Originally Posted by Aaronhl View Post
    If he runs in Miami salt water - good luck getting those power heads studs loose in a few hours without much damage to put everything back on lol
    Very true

  15. #10
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    Update.
    got a mechanic to come by and see the boat. He noticed a small leak in the powerhead gasket so he took it apart to replace. While removed, he looked over the exhaust plate and motor mounts and claimed there were no leaks.
    The engine is definitely easier to start now but it still smokes from the lower cowling/steering side and does not want to idle on the water. Runs beautifully outside of the water and will run beautifully above idle on the water.
    Will be doing a link and sync next on the water to see if that helps.
    Starting to think the midsection was badly tampered with by a previous owner. Causing these problems.

  16. #11
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    Is the exhaust for sure coming from the plate area? Usually these type leaks are obvious when tore apart, and mechanic said he saw no leaks. Make sure it’s not a problem with the rear exhaust reliefs leaking into the cowl itself maybe. If so the exhaust in the cowl being drawn back into engine will cause it to run poorly. Maybe see how it runs with cowl off, it runs ok it’s just the exhaust going back into engine, if no change then link and sinc may be in order, like needing a little more idle timing when in the water.

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  18. #12
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    Thanks for your response I think you’re on to something! I will attempt taking the cowling off next time I’m on the water. The lower cowling is actually from a newer efi 2.5l so maybe the rear exhaust reliefs are missing an adapter or exhaust boot or something. I have never actually noticed exhaust come out of the rear of the lower cowling where the exhaust reliefs should be. Only at the front steering area and at the bobs exhaust and prop.
    Last edited by Toeknee217; 01-16-2022 at 09:55 PM.

  19. #13
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    I was wondering about the possibility of cowl to mid/adaptor combo. If the exhaust is going into cowl instead of plumbed out of it, it would idle bad and will stumble coming off idle.

  20. #14
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    Quote Originally Posted by FORBESAUTO View Post
    I was wondering about the possibility of cowl to mid/adaptor combo. If the exhaust is going into cowl instead of plumbed out of it, it would idle bad and will stumble coming off idle.
    So I took apart the lower cowl and found the midsection has the two rectangular exhaust reliefs underneath the newer cowling. The mechanic had mentioned the exhaust reliefs would be deleted once the bobs machine exhaust housing got added? Not sure if that is true.
    I will start the boat up later today and see if any exhaust comes out the reliefs, if it does I believe we found the problem and I will need to change out the cowlings back to the original 1995 vintage ones that leave the exhaust reliefs open.

  21. #15
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    Looking like we got this coon treed. If ya got the open eyebrows dumping into the new style closed lower, that’ll do it. When on the water you’ll always get exhaust out the upper reliefs. Depending on your cowl, but most of the later ones used a rubber hose from mid that sealed to lower cowls and the slots where not open. Probably would be easier to put a cowl on it that leaves the slots open below the cowl.

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