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Rough Idle, Fine above 1000 RPM

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  • Rough Idle, Fine above 1000 RPM

    My 97 Glastron with a 5.7L GL has developed a very rough idle, sometimes stalls. As soon as I hit 1000ish RPMs, runs like a top. Spark plugs were fouled out, so I replaced them. Was better for a few runs but doing it again. Cap, rotor, plug wires have all been changed in the past 2 years. Holey 2 barrel carb has been rebuilt a year ago. Fuel/Air mixture adjustment has been tweaked and didn’t help. The Choke plate is open and not stuck. Someone mentioned replacing the Idle Air Control Valve (IAC). Any thoughts?
    '90 Regal Commodore 360. Twin Merc 7.4L Inboards
    '97 Glastron GS205 Open Bow Volvo 5.7L Cobra
    GONE - '89 Condor Mach 1 2350, Mercruiser 7.4L, Bravo
    GONE - '87 Sea Ray Pachanga19, Mercrusier 4.3L Alpha

  • #2
    Carb doesn't have a IAC, you need the carb taken apart, cleaned real good. The idle circuits are clogged up
    94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
    95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
    07 Seadoo GTX
    Merc Adults Only VP Adults Only
    G-Dad always said "First Liar doesn't stand a chance"

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    • #3
      Agree, IAC is a TBI fuel injection device.

      Its also appropriate to look for vacuum leaks, there aren't many paths on the Penta setup but the gasket under the carb could possibly be a source that tampers with idle air/fuel balance.

      If I clean the carb, I like to buy a "1 gallon paint can" of carb cleaner that's actually about half-full at the auto parts store where you fully dunk the carb in overnight and it has some special solvents which goto work on the gum very effectively. Then I like to use the wire from a paper wire-tag and poke thru all of the passages, very carefully, but to make sure they are all open. Finally blow clean with compressed air and reassemble + test.

      Before you get thru that, some Sea Foam in the gas can potentially help but its more of a proactive maintenance item in my mind than a magic bullet which is going to solve all of your problems. But, for $6 (around here) its a fast thing to try with minimal effort.

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      • #4
        Seafoam
        94 Formula 27PC Custom 509MPI MEFI3 , B3 XR
        95 Rinker 232 w/ 7.4L Carb 0F425011 B1 0F486471
        07 Seadoo GTX
        Merc Adults Only VP Adults Only
        G-Dad always said "First Liar doesn't stand a chance"

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        • #5
          Test: can of starting fluid, start motor, spray around the intake side, carb area.
          Make any difference? sucking air.


          Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
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          In Scenic , Beautiful San Sebastian by the sea Florida

          88 Bayliner 21CC 2001 Mercury 225 HP EFI

          PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM

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          • #6
            Ayuh,..... Foulin' the plugs is too much fuel, rather than to much air,.....

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            • #7
              first thing to check.

              does the clear line from the fuel pump to the motor have fuel in it. if it does, replace your fuel pump

              if not, go back to the carb pull it apart, and chase every passage. replace your power valve.

              you have either fuel entering the motor externally, or you have an issue in the idle circuit.
              Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

              1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

              Past Boats
              1970 Wooster Hellion - Merc 9.8
              2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
              1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

              What’s behind you doesn’t matter.Enzo Ferrari

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              • #8
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                Doubt it would be the source for too much fuel like your speaking of, but I'd also check cap and rotor again, even if it's been replaced in last year. Once had a motor with similar symptoms. Corrosion on 1 yr old cap contacts was enough to cause ruff idle. Once rpm increased, charge was able to navigate through the corrosion and idle issue went away. A new cap and rotor fixed it. A 2 minute easy check for piece of mind.

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