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Mercruiser 454 flatspot at 3200-3800 RPM

Discussion in 'Technical Discussion' started by Stardust, Sep 27, 2011.

  1. Stardust, Sep 27, 2011
    #1
    Stardust

    Stardust New Member

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    I just put my engine back together. (valve work & exhaust). Ran ok before, now as I accelerate it runs strong, at 3200 it begins to get flat. I have to push the throttle all the way up, then it picks up to WOT at 4200 and runs great. Checked timing @ 8 degrees, carb is adjusted right.

    Any ideas how to troubleshoot?
  2. Capt J, Sep 27, 2011
    #2
    Capt J

    Capt J Senior Member

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    Check total timing at rpm's, could possibly be valves floating (weak springs), could be the carbuerator isn't getting vaccuum and the secondaries aren't opening.......
  3. Capt Ralph, Oct 17, 2011
    #3
    Capt Ralph

    Capt Ralph Senior Member

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    Any follow up?
    I kinda thought vacuum leak or carb float level.
  4. Stardust, Nov 28, 2011
    #4
    Stardust

    Stardust New Member

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    I'm pretty convinced I need a carb rebuild. Engine runs fine up to about 3000 rpm then it bogs untill full throttle at 4200 rpm.
    I checked that the choke is opening and not restricting the 4 bbls, timing is right. I will double check with the mechanics at my marina.

    Being as it operates fine at low rpm's, it has to be related to the high speed jets I would think.
  5. NYCAP123, Nov 28, 2011
    #5
    NYCAP123

    NYCAP123 Senior Member

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    Being a firm believer in the KISS principle check the vacuum as mentioned earlier, and just to eliminate the stupid, check that your throttle cable is clipped in place. Had that cause similar symptom twice. Hate to see you go for a rebuild if it's just a hose or a cabletie.
  6. Capt J, Nov 28, 2011
    #6
    Capt J

    Capt J Senior Member

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    I would check fuel pressure as well, and the accelerater pump on the carberator if it's equipped with one.