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  1. #1
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    25 hp mercs XS, XD, SeaPro/ Marathon...

    I dreamed of owning a 25xs for years and looked around but could never find one in decent shape...and I am still looking.

    After getting discouraged by my failed search I started researching what exactly made them different from the production motors. Anyone who knows a little about them knows they ran the same block, different pistons in some of them and fishin motor pistons in others, short mid, different exhaust plate, racing lower, restrictor plate behind the carb, different throttle plate, smaller flywheel in some and fishin flywheel in others, and some other miscellaneous differences. Anyway, most all the parts that are unique to the 25xs are impossible to find as not many were made and the people who have them are likely still racing them.

    I decided to just start building a 25 merc that had some of these minor and "bolt-on" mods that the 25xs had. I didn't try to conform to any class rules or that sort of bs, I just wanted to build a cool 25 hp. It led me into a stupid addiction that I have since failed to control. Anyway, I decided to post some pics on here to see if anyone else has anything to say. I am not one for technology or social media, even though I know it can be useful for sharing info and building crazy stuff. The only other place I post this stuff on is Instagram and that I have barely figured out. If you any of yall wanna see more of my pics before I get around to posting them on here you can look me up on IG and guess what, my name is the same @25two.stroke .

    Anyway, this is the second 25 I built. It has a dry side pipe where ALL the exhaust dumps. I did this so that I can run acetone and other form of alcohol fuels and it wont melt the paint off my lower or mid. As with all my projects, they are never DONE, I'm always tweaking and makin stuff for them.

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    8' Yellow Jacket - 25 hp Mariner
    1984 Hst VKing - 15" Bridgeport EFI
    1990 Collins Mirage - 15" 260
    22' Aquasport - 150 SeaPro Fourstroke

    25 hp Merc that runs on ACETONE
    25 hp Merc with dry side pipe
    25 hp Merc with 11" mid and 2.42:1 gears


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  3. #2
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    Some clarification right after I posted this. The red bacon stripe motor is the side pipe one. Same one that has the velocity stack on the carb. The cowl is fitted with 25xs pull bungees and the velocity stack is cleanly cut through the front of the cowl. I'll take some better pics of it if there's any interest.
    8' Yellow Jacket - 25 hp Mariner
    1984 Hst VKing - 15" Bridgeport EFI
    1990 Collins Mirage - 15" 260
    22' Aquasport - 150 SeaPro Fourstroke

    25 hp Merc that runs on ACETONE
    25 hp Merc with dry side pipe
    25 hp Merc with 11" mid and 2.42:1 gears


  4. #3
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    I am thinking of building one for a small hydro.......will be using a old chrysler lower unit with 1.62 gears. And using a old SSM prop .......Thinking of putting the Merc power head on the lower unit for low center of gravity. No exhaust housing..... less weight....

  5. #4
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    You got a pic of the Chrysler unit you wanna use? If you need a driveshaft for splines I may have a spare one...I had to make a few shafts for putting four stroke gears in these cases. You planning to build a skeleton mid for one? You thinking chrome block or nik?

  6. #5
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    Here are some more pics from fabricating my 11" Mariner mid. Cutting and welding was done at Calhoun Marine. Also, some pics of a few lower unit mods I've done and some fire props from Hill Marine. I can stare at those little beauties for hours.
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  8. #6
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    Sideways pics...heres a water pressure gauge and fuel pump/ignition kill switch plate on the tiller handle. Rigged this motor with an electric fuel pump inside the cowl (you can see it in the pic of the motor on an aluminum boat I was testing it on) among other mods like above-water exhaust.

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    8' Yellow Jacket - 25 hp Mariner
    1984 Hst VKing - 15" Bridgeport EFI
    1990 Collins Mirage - 15" 260
    22' Aquasport - 150 SeaPro Fourstroke

    25 hp Merc that runs on ACETONE
    25 hp Merc with dry side pipe
    25 hp Merc with 11" mid and 2.42:1 gears


  9. #7
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    No pixs yet still in the thinking stage.... planing on using a old Chrysler 35 hp lower unit........Just a cheep build for a friends grand son........

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  11. #8
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    Here's my 11" mariner nearly done. Its a nikisil block with some light interior mods like pistons/rods and transfer port cleanup. Bumped up the timing, carbon reeds, re-jetted and cross drilled the jet inlet. Lower is coned and has brand new 4stroke gears.
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  13. #9
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    few ph pics when I put a charge coil on it to drive a proper tach....

    Have been looking for years to find somone willing to cut a flywheel for me. Thats pretty much the last mod I'd want to do, but no one is willing to try. I have like 6 or 8 cores to mess with, just need someone able to turn them down and balance them. Wouldn't even really need to drill them, theres enough spare material around the perimeter to loose a few lbs there...

    Don't need timing marks cause they run max mechanical timing and base is set by where they run good...and you can always throw a stock on there to check timin. Let me know if any of you know one who would be willing to mess with cutting a cast flywheel.

  14. #10
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    I am on the hunt for a 25 XS
    Going to use it on a C class Hydro

  15. #11
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    Very cool motors 25two.stroke I have a couple of the 18-25 fishing motors and I am not very familiar with these motors. I always felt the mids and lowers were overbuilt and heavy. I like the cutdown mid that you had Calhoun do. Is there a resource for the differnces between the 18-25 fish motors and the racing 25XS? Someone needs to make a shopping wish list or a spreadsheet with years and part numbers of good parts.
    Jason
    Outboard Junkie


  16. #12
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    I have raced the 25xs back in the day and still have one. My question to you is i have never dealt with the mercosil blocks, only the chrome. They are impossible to buy new and cant be bored or sleeved(economically). Have you compared both, port wise, and performance wise? I have one that needs a bore, i was thinking about going 30 over. In the classed we race, there are port specs so it would have to be really close to the chrome bore block. What is the piston to bore clearance of the mercosil block. Best i can tell by the manual is around 5 thousands? The WSM rebuild kits are pretty cheap and have everything to rebuild. Just wondering your experiences with the mercosil block. Thanks

  17. #13
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    I don't have all the answers for you, but I do know some things.
    APBA rule book has the port specs for both the mercosil and chrome blocks. As far as I know any race class will let you run either block cause there ain't much different spec or power wise. They are real close, but not identical. You can go 30 over in the mercosil blocks but not many shops have the diamond hone required to finish the bore. I have three or more mercosil blocks that need to go 0.030" over but its not really economical even for those, unless you found a motorcycle shop local that happened to have the boring stuff for that size.

    The mercosil blocks burned up WAY, and I mean WAAYYYY, more often than the chrome. They came with a 0.076" main as opposed to the 0.080 in the chrome motors...that burned them up whenever they over revved or started losing their water pump (emissions regs pushed that jet change). The pistons were identical dia. between the chrome and mercosil blocks and I believe the piston to wall clearance was 0.003" to 0.005"

    The mercosil blocks, if you find one thats not burned up yet, are nice and powerful once you get a 0.080 jet in there and clean up all the burrs down the intake ports. If you do build one 0.030 over I would bore both holes out. My $0.02

  18. #14
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    Hydroo, would a 402X work???
    Attached Thumbnails Attached Thumbnails 402 X (2).JPG  

  19. #15
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    Quote Originally Posted by SlingShot View Post
    Hydroo, would a 402X work???
    Thats a D Class motor I believe, not C.
    Cool motor though, how much would you let it go for?

    BTW carb a rator I have a NOS chrome bore 25 block...
    Last edited by 25two.stroke; 10-16-2021 at 03:14 PM.

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