Store Links Mobile - Shop Now


1 of 2 < >


The iBoats Forums will be down for maintenance starting at 1 PM MST on 12/16/2017. The estimated amount of downtime is 3 hours. Thanks for being a part of the iBoats community!
2 of 2 < >

Help Tip: If you have a question that has not been answered to your satisfaction in the archives, it is always best to start a new thread of your own. By starting your own thread, you will receive the maximum number of views by forum members.

Below are some additional forum policies in hopes of all iboats members will follow, Thank you.

1. Please do not reply to old topics or hijack existing topics. Old topics of a technical nature are like a library book, Please do not write in them.

2. Old topics should be considered archives and used for reference only. Please do not reply to them.

3. Do not take over someone elseís topic (aka hijack) with your own question, even if it is similar. If you have a question that has not been covered to your satisfaction in the archives, it is always best to start a new topic of your own.

4. If you have a question for the original poster (OP) and the topic is over 30 days old, send the OP a PM, he may not even visit the forums any longer, or may not notice your question in the old topic.

5. By starting your own topic, you will receive the maximum number of views by forum helpers that may not even notice your question when itís posted at the end of someone elseís topic. And those answers will be specific to your particular issue.

6. Please do not post to topics that have been inactive for more than 3 months unless you are the original poster. We have very active forums and any topic that remains inactive for that long should be considered "dead". It is especially confusing when there is an entirely new question posted to an old topic.

7. Posting at the end of any topic is considered to be hijacking the original posters topic which in turn subjects the topic to be closed if it continues to happen thus not making it fair to the original poster in the future had for some reason he/she needed to return for additional information or provide an update of the problem solved which is always welcomed within a reasonable amount of time frame.

8. Please note that you should see a red banner pop up near the bottom of each inactive topic asking you not to reply to old topics. The Red banner will read: Please note this topic has been inactive for 90 days. For the best results, please start a new topic.

Thank you all in advance for doing your part in helping iboats run a smooth ship.

Additional forum rules linked below.
See more
See less

Mercury 25hp running rough

  • Filter
  • Time
  • Show
Clear All
new posts

  • 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
    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
      Re: Mercury 25hp running rough

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


      • #4
        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
          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
            Re: Mercury 25hp running rough

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


            • #7
              Sign up today
              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