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  1. #1
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    Combustion chamber size

    I am changing heads on my 225 pro max the original heads combustion chamber measures 2.423 in diameter and the other two sets measure around 2.296. the other sets of heads are milled to 33 cc is this why? are they ok to use?

  2. #2
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    MY mistake the smaller chamber heads are 34 cc anyone?

  3. #3
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    That would be fine if you use good gas.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  4. #4
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    Go ahead and check compression with a fresh battery, but 34's and 91 are par for the course for a promax. With 34's and a fairly worn promax my compression is 139-142, with 150 being the limit for 91 octane. Might be worth checking which head gasket thickness you are using, there are 0.75mm, 1mm, and 1.2mm, which will have an effect on compression.

  5. #5
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    I will check the gasket thickness compression was a even 130 see what it is with new heads previous owner had too many spark plug threads stripped on factory heads coil in one needs more thanks everyone for the help!

  6. #6
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    My standard build on 225 Pro Max consists of a 34cc head utilizing a 1mm head gasket , at sea level will yield 138-142 lbs compression with fresh rings and bores which is safe with 91 octane with no more than 25 degrees timing ..

    good luck ,
    Jay @ JSRE
    Jay @ JSRE


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  8. #7
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    Btw , to repair damaged spark plug threads I use only Time Serts for ALL my thread repairs works great for 20 years
    Jay @ JSRE


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  10. #8
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    Thanks for the info I put the 34 cc heads on today the timing is set to 24 degrees the boat ran great! Should I go to 25 degrees? the motor has unknown hours but was rebuilt about 6 years ago.

  11. #9
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    Will help bottom and mid range a smig I like it a bit safe at 24 ��
    Jay @ JSRE


  12. #10
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    I’d spend the time to dial indicate ALL holes shoot timing at full advance on ALL holes find your “high” hole and use it as your constant, call and I’ll explained
    832 597 2603
    Jay @ JSRE


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  14. #11
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    I will do than thanks Jay.

  15. #12
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    Quote Originally Posted by Jay Smith View Post
    I’d spend the time to dial indicate ALL holes shoot timing at full advance on ALL holes find your “high” hole and use it as your constant, call and I’ll explained
    832 597 2603
    good to see you back in here, Jay
    1990 Shadow bass boat w/ 2.4 200 Merc. Totally resto'd boat and love it!

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  17. #13
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    Jay sells a nice dial indicator for setting up our motors on timing.

    Quote Originally Posted by Merc Speed View Post
    I will do than thanks Jay.
    1990 Shadow bass boat w/ 2.4 200 Merc. Totally resto'd boat and love it!

  18. #14
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    TY glad to be back helping out with 2.5 issues …

    Jay @ JSRE
    Jay @ JSRE


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