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Yamaha powerhead on my Merc . . . a question

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  • Yamaha powerhead on my Merc . . . a question

    I've got a Mercury 30hp 4 stroke that has a carb'd 3 cyl Yamaha powerhead, it's a couple of years old. Question is: I believe this engine is detuned from 40hp since it is identical in all specs to a Merc/Yam 40 except the power rating. Anybody know what it would take to pump it back up to 40hp? Carb jets maybe?

  • #2
    Re: Yamaha powerhead on my Merc . . . a question

    "all specs" covers a lot of unpublished territory, Chaps.The most common way of "regulating" output of small 4 strokes is venturi diameter in the carb. Next might be cam timing and lift or exhaust geometry.Check the parts manuals for both engines. Where part numbers are different, the parts are different.In the end, you have the same displacement and probably wouldn't notice any significant boost in performance.
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    • #3
      Re: Yamaha powerhead on my Merc . . . a question

      Not sure what prop you are running, but performance can often be enhanced with a change in prop pitch. Are you turning the correct RPM at WOT? There is a lot of info regarding props in the forums. Just something to consider.

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      • #4
        Re: Yamaha powerhead on my Merc . . . a question

        Thanks guys . . .By all the specs I mean everything I have seen published relative to displacement and other physical specs that the mfgr shares in their sales literature. A more thorough inspection of the parts manuals would certainly tell the story though I posted the question here in the hopes that someone might know the quick answer. I'll be heading over to the local Merc shop to see if I can look thru the books.

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        • #5
          Re: Yamaha powerhead on my Merc . . . a question

          The cost for all the parts to convert would almost certainly be greater than if you just trade it in for what you want. And if you have any warranty left ... say goodbye. You would notice a little more speed (a very few mph) and a little faster acceleration.It's a Mercury powerhead with Yamaha internals and certain bolt-ons. Trading parts between brands may prove frustrating.If trading is a possibility and you're limited to 40Hp, then consider the 4-cylinder Merc. There's the 30/40 3-cylinder, or the 40/50/60 4-cylinder. You would notice a bigger increase in power and acceleration moving up to the 40 4-cyl than the 40 3-cyl.
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          • #6
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            Re: Yamaha powerhead on my Merc . . . a question

            Problem solved . . . the only difference between the two engines is on the 30 the isolator plate between the carbs & the intake manifold has a smaller set of holes and is acting as an air/fuel restrictor. Thanks TMD and Merc parts book. Fourteen bucks for 10hp ain't too bad.

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