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  1. #1

    Default Mercury 25hp running rough

    Hi guys I was wondering if someone could put me in the right direction as to what could be the problem with a Mercury 25 hp 2 stroke outboard motor.
    Now before I start I'm mechanically minded and I'm a Auto Electrician by trade.
    I purchased this 1985 outboard from eBay a while back and they said that it was in good running condition ( Yeah right ). Upon starting it does idle but shakes excessively like missing a cylinder. When I rev it up it stutters and then evens out but revs go really high ( checked both spark outputs and both cylinders are firing). Compression was cylinder 1 =150psi and cylinder 2 = 175psi, upon strip down of motor I found carbon blockage of exhaust relief holes causing high compression on that cylinder. After cleanup of engine and new gasket kits fitted I also fitted some racing reeds that I got cheap.
    Now my first instict was that it had carb issues so I ordered and fitted a new repair kit for the tillotson carb type Bc1b. Now with it all back together and running, same problem is still there. Compression on each cylinder is now 150psi exactly. Engine shakes and as you rev it up it splutters then revs it's guts out. I'm running engine with prop in nuetral and ear muffs for water cooling.
    Would it be that it needs prop to be running for load? As I'm not familiar with these types of engines ( give me a complexed EFI system any day ) . I also have a similar engine 15hp Merc 2 stroke which runs perfectly no shaking and smooth idle. Any ideas? Regards Dino

  2. #2
    Petty Officer 2nd Class
    Join Date
    May 2009
    Long Beach, WA

    Default Re: Mercury 25hp running rough

    Good for you.. you checked the compression. Now you mentioned that there was an exhaust blockage from carbon. That tells me there as an issue with this motor previously. How far you took it down is next. Reeds are on the outside of the powerhead as I remember. Did you pull off an exhaust cover and check for further blockage of carbon? If there is good spark and you have clean fuel delivery, (compression is good) then the timing and synchronization process is next. Adjust that per spec and away you go.

    I'd be looking into the carboned up issue. That problem needs to be discovered for cause and remedied.

  3. #3

    Default Re: Mercury 25hp running rough

    Engine was stripped and only piston and crank was left untouched still mounted in cylinder block.

  4. #4

    Default Re: Mercury 25hp running rough

    Hi , I pulled it all down except for the pistons and crank so exhaust and intake has all been cleaned. I do think it might be timming or sinc issue. I can't seem to find any info on this particular motor as one manual I downloaded engine does not look the same as pictured. The carbon issue seems to be due to old age as it seems not to have been touched from new. The blockage was a small .5cm hole that goes directly from exhaust box to cylinder chamber ( looks like a relief port or something of that nature ), this was full of carbon and completely blocked ( being so small of a hole I don't think we need to go into the issue of why it was blocked up ).
    The reeds sit outside the cylinder block bolted directly behind the carb. Spark is good and I have removed each lead and motor does respond to each lead being disconnected. With fuel issue I have removed and stripped down carb, cleaned all ports and replaced parts with kit provided including new fuel pump diaphrams. Motor info is as followed 0G021158 and on block 6153848. Regards

  5. #5

    Default Re: Mercury 25hp running rough

    So far I have downloaded one manual and purchased a complete Mercury service manual CD but both do not show my type of motor. I have found parts listings that are the same as mine and they regards my motor as USA type 25 hp 2 cyl falling in between models 5705532-6416712. Any help for ignition timming and sinc for this model would be greatly appreciated. Regards

  6. #6

    Default Re: Mercury 25hp running rough

    This is my mini speed boat that the motor will be replacing the 15hp already installed. Cheers
    Attached Images Attached Images

  7. #7

    Default Re: Mercury 25hp running rough

    Ok I got the problem sorted out , I hooked it up to my Oscilloscope and found braking down trigger to coils ended up being faulty control box. Thanks to "G" for being the ony one who responded to help. Regards

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