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Mercury 25XS Jetting

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  • Mercury 25XS Jetting

    Hi All.
    I've Had several successful runs in my Bezoats shovel now, and am pleased with the results. I haven't crashed, or broken anything yet! Must have done something right. It wouldn't have happened without the help from the HR community. Thanks to all.
    My next question is regarding carb jetting on my 25XS. It certainly appears to be running quite rich. A lot of burbling in the mid range and accelerates slugishly.
    Now, I realize there are soo many factors to consider here, but I am looking for a baseline.
    My carb currently has a .086 jet in it. Plugs are wet and black. The parts diagram on the Crowley site for the 1985 (ser # 85A000 and up. Mine is 85A006) specifies .080. Another member thinks the .086 is stock correct. I already purchased a couple .080's, but don't want to smoke my powerhead.

    While I'm here, what is the best spark plug for these motors? it is hard to get a good reading with the surface gap BUHW plug?

    I promise to get some more pics and videos loaded up in the near future!


  • #2
    Hi Tom,
    I have a .084 jet in my 25xs with B-9ES plugs usually available at CTC. I would change your plugs before changing the jetting
    and see how it runs. Regards Michael


    • #3
      To get jetting right you need to also pay attention to max ignition advance the engine is set at. It is better to be on the slight rich side with jetting, the engine will live longer and detonation will have less of a chance provided you have the max advance set right. Are you familiar with reading spark plugs to give a visual on air fuel ratio? There is more to it that color. With jetting and spark advance always safer to start out rich and less advance and sneak up on it. Do you have a accurate tach on board to note peak rpm and any rpm drop off at full throttle over a reasonable distance I used to use about 3000 ft? Also the oil fuel mix ratio and octane plays a lot in the setting up of jetting, timing, etc. Its a fun trial and error and testing (never alone) game and props have a lot top do with it along with the boat set up. Fly them aired out and keep them right side up, but with it all have safe fun ;-)
      "Keep Move'n" life is catching up!
      No man's life, liberty or property are safe while the legislature is in session.


      • #4
        What fuel ratio? Try about 10 or 12:1. And don't go leaner than a .084 jet unless you're above 3,000 ft. And , yes, most importantly get rid of surface gap plugs and go to electrode tip very first thing!


        • #5
          Thanks for the tips. I will try the electrode tips to get a proper plug reading before jetting down. I still need to get a tachometer. Fairly confident I am not going north of 6500 yet. Still getting used to driving this thing. Haven't let it really air out yet. Totally different animal than anything I've ever driven. My heart starts to beat faster just thinking about it!