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  1. #1
    Old&Slow's Avatar
    Old&Slow Guest

    Default 1992 Mercury 25HP random misfire

    I have a 1992 25HP two cylinder 2 cycle Tracker by Mercury outboard in generally excellent condition. It has been stored for 9 months while I was recuperating from surgery. It now has a random miss at all rpm ranges. At idle it lopes like it has a hot cam in it.

    Compression is 135-140 on both cylinders. Spark appears strong on both cylinders. New plugs had no effect. I hesitate to think it is a fuel problem because the misfire occurs at all rpm ranges. Fuel is fresh, never any ethanol, and the filter looks like brand new.

    I would like to try diagnosing the problem myself before resorting to take it to a shop (there aren't any locally), and would appreciate any thoughts from those who have experienced this issue.

    Thanks!

  2. #2
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,691

    Default Re: 1992 Mercury 25HP random misfire

    Buy one new coil and see if that's it. Try it in each spot.

    Jeff

  3. #3
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Jeff, it has a nice hot spark on both, but I can see how it might have a random miss I cannot see just by spinning the engine. Therefore, coil on order. Thanks for the tip & I'll check back here in a week to see what happens.

  4. #4
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
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    9,131

    Default Re: 1992 Mercury 25HP random misfire

    Coils are a very common issue on these older portable models. They get cracks in the casing (looks like spiderwebs if you clean them up well enough to get a close look).

    Sometimes you can hear the (random) "snap" with it running with the hood off. If you run it on a flusher or in a (tank) at night with the hood off you will get the visual.

    If these are original coils you have done well - anytime after 20 years old I tend to suspect the coils with similar symptoms.

    If you find yourself in need of a second coil (or want another spare - personally I would change both if this is in fact your issue) I would go with the coil from Sierra instead of the OEM. I have personally found them just as good (if not a little better - particularly in the 15 horse and below offerings), and you save a few bucks to boot.
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  5. #5
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
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    Default Re: 1992 Mercury 25HP random misfire

    A bad coil will still make spark cranking over, but fail at high rpm.

    Jeff

  6. #6
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Coil(s) are not the problem. I got a replacement coil and it makes no difference in either position. The engine still misfires badly at idle and randomly misfires at higher rpm. What else can I check before taking it to a shop? As previously stated, fuel is new, filter is clean, compression is good. Engine has not been molested (yet). I have most tools and a lot of engine experience, just not on outboards. Thanks!!

    PS - I do not think it is a fuel issue, but can the carburetor cause a miss at idle and at higher rpm? I don't think this is a likely scenario, but perhaps?
    Last edited by Old&Slow; 07-15-2018 at 05:39 PM. Reason: add comment

  7. #7
    Join Date
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    Default Re: 1992 Mercury 25HP random misfire

    Fuel issues tend to make "slow" misses, where ignition issues are "fast" misses.

    Seeing it's not the coils (sorry), and assuming there are no bare or breaking wires, then I'd look into another switch box. Seeing I've already wasted some of your money, I'd recommend buying a used one on eBay and giving it a try. If it cures it, you're good to go; if not, you'll save a bunch of money.

    Jeff

    PS: It might also be the stator or trigger coils, but these you can visually inspect and checked (for resistance) with a meter. Not so the switch box, so do this inspection first.

  8. #8
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Thanks, Jeff.
    You haven't wasted my money, but I appreciate the thought. I will pop the flywheel and do an inspection. Also will look into the switch box. Thanks for pointing me down the path.
    Mike

  9. #9
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Old & Slow is me -- the engine is actually a 1999, but I still have the same issue. I got a new switch box, when I removed the old one I found a hidden ground wire that had been spliced with a crimp. I just knew that was it, but no. Swapping the switch box did not help. I still have a miss throughout the throttle range. I put an infrared thermometer on the plugs and the bottom one is about 5 degrees colder, so I'm guessing that is where the miss is. The plug wire reads zero resistance, so I believe it is OK. Not coils, not the switch box, I guess the flywheel is coming off. At least I know several things that are not the problem. When I do get it sorted out, I will post. Thanks!

  10. #10
    Old&Slow's Avatar
    Old&Slow Guest

    Default Stator?

    I checked the stator resistance per the CDI troubleshooting guide and got a reading of 404 ohms. The CDI guide says OEM should be 660-710 ohms and CDI 350-450 ohms.

    It appears the stator is out of spec, but before I break out the credit card, would this cause a continuous miss?

    Disconnecting the rectifier per CDI guide made no difference, so I'm removing the rectifier as a cause (for now). I've replaced coils and switch box, so those are not the problem.

    Thanks for further help on this.

  11. #11

    Default Re: 1992 Mercury 25HP random misfire

    Have you looked at the carb? What is your compression? Is the stator.rod still attached to the block tightly? That stator should not move at all.
    Last edited by flyingscott; 07-28-2018 at 11:49 PM.

  12. #12
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Compression is 135-140 on both cylinders. I still don't to think it is a fuel problem because the misfire occurs at all rpm ranges. Fuel is fresh, never any ethanol, and the filter looks like brand new. I don't understand about stator rod. The stator is bolted down and does not move. The trigger coil moves with the throttle, and everything is properly connected. Visual inspection of coils and magnets shows no cracking, no burnt coils, no loose magnets, etc.

    The engine has a consistent misfire from idle throughout the rpm range. Both cylinders are firing - I can pull either plug wire and the effect is very noticeable. The engine starts right up, but always the same misfire. Coils and switch box were replaced with no effect.

    Using the CDI checklist, both the stator and trigger are out of spec for resistance but within spec for DVA (disconnected). When I tried to do DVA (connected), I could not get a steady reading, so I must be doing something wrong. I'm going to contact CDI in the morning & see if they will help me.

    I believe I'm down to either the trigger coil or the stator at this point.

  13. #13

    Default Re: 1992 Mercury 25HP random misfire

    why dont you put.the serial # up?. You seem to have some confusion on the yr of the motor. Is it a 92 or 99 model? They have very different ignitions.

  14. #14
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Mercury Tracker
    Model 1999 25EL
    Serial: OG829921

    I tried to change the title of the thread, but was not successful. Boat is a 1992, engine is a 1999.

  15. #15

    Default Re: 1992 Mercury 25HP random misfire

    I do not believe a 1999 motor would have a trigger that moves.

  16. #16
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Quote Originally Posted by flyingscott View Post
    I do not believe a 1999 motor would have a trigger that moves.
    Thank you for the assistance with my issue. Maybe I am not using the right terminology. This is what I call the "trigger." It rotates and sits underneath the other coils, which I have been referring to as "stator."


    Click image for larger version. 

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  17. #17

    Default Re: 1992 Mercury 25HP random misfire

    Is your motor a jet? That is what the # comes up as. I do not believe that should move the later model motors had the timing curve in the switch box.

  18. #18
    Old&Slow's Avatar
    Old&Slow Guest

    Default Re: 1992 Mercury 25HP random misfire

    Quote Originally Posted by flyingscott View Post
    Is your motor a jet? That is what the # comes up as. I do not believe that should move the later model motors had the timing curve in the switch box.
    Sir, I'm not sure why you keep insisting - this is the part that is in my engine. I'm sure you will have the last word, but it no longer matters as I will be leaving this forum.

  19. #19

    Default Re: 1992 Mercury 25HP random misfire

    Good luck with your motor I am sure you will find.someone who tells.you what you want.

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