7.4 454 running sludish at high rpm

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by ynot, Sep 15, 2019.

  1. ynot

    ynot Member

    87
    Feb 14, 2010
    s ca
    400 sedan bridge
    454 mer
    hello boaters
    I have 2 7.4 454 Mercruiser big block
    at idle looks and sound good
    then when in gear up to 1500 rpm no issues then anything over no power at all
    need assistance if anyone experienced same issue
    thank you
     
  2. tuttee1948

    tuttee1948 New Member

    1
    Aug 1, 2016
    Cayuga Lake, New York
    1988 340 Sundancer
    Twin 'gas cooled' 454 V8's with V Drives
    I had similar problems with my twin 454's. After much screwing around, I installed new plug wires and it made all the difference in the world. I also had new plugs installed, tuned them up and have been enjoying great boating for 3 years.
     
  3. hottoddie

    hottoddie Well-Known Member

    Jan 11, 2012
    Boston/Cape Cod
    1986 Sea Ray 390 EC
    Garmin 4212 Chartplotter
    Garmin 24 HD Radar
    Garmin 546s Plotter
    454 Crusaders
    Is this a new problem? Did it begin to happen to both engines at the same time? one or two fuel tanks? If in salt water how long has the boat been sitting in the water and how often is the boat used? Could be the running gear and bottom is badly fowled.
     
  4. SrtThis

    SrtThis Member

    72
    Feb 20, 2019
    Maryland
    270 SE sundancer
    7.4 MPI w/bravo III
    carb or injected?
     
  5. ynot

    ynot Member

    87
    Feb 14, 2010
    s ca
    400 sedan bridge
    454 mer
    injection
    thank you
     
  6. ynot

    ynot Member

    87
    Feb 14, 2010
    s ca
    400 sedan bridge
    454 mer
    both engines, checked fuel looks ok ,and replaced fuel filters .
     
  7. Steve S

    Steve S Active Member GOLD Sponsor

    991
    Jun 5, 2007
    Northern IL.
    2000 400 Sedan Bridge with twin CAT 3116's

    2000 340 Sundancer - SOLD!
    210 Monaco 1987 - SOLD!
    Twin Caterpillar 3116's 350 HP straight drives
    A 400DB is a big heavy boat for gas motors. Describe sluggish. What is your cruise speed @ what RPM.
     
  8. ynot

    ynot Member

    87
    Feb 14, 2010
    s ca
    400 sedan bridge
    454 mer
    than you Steve
    I don't use the boat a lot
    I just like to stay at the marina and chill
    the fuel is old but looks good after test
    on idle both engines sound good even at 2400 rpm no problem
    when I take the boat out ,it runs good at around 1200 rpm then when I push on throttles and reach little over 1500 then both engine kind choking with no power what so ever to plain .
    few month ago I reached 4500 rpm with good speed
    thx again
     
  9. SrtThis

    SrtThis Member

    72
    Feb 20, 2019
    Maryland
    270 SE sundancer
    7.4 MPI w/bravo III
    check your fuel pressure...
     
  10. hottoddie

    hottoddie Well-Known Member

    Jan 11, 2012
    Boston/Cape Cod
    1986 Sea Ray 390 EC
    Garmin 4212 Chartplotter
    Garmin 24 HD Radar
    Garmin 546s Plotter
    454 Crusaders
    I would either haul the boat to be pressure washed or hire a diver for a hull inspection. After that plan on running the boat to get the fuel tank below 1/4 tank and put in a couple of hundred gallons of fresh stabilized fuel.
     
    Snapon24, Chris-380 and techmitch like this.

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