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Loss of power / running hot-gctid823900

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    Loss of power / running hot-gctid823900

    1989 460 Cobra

    I overheated the motor because of a impeller problem.

    We did a compression test and all was well. Then we did a full tune up (plugs, points, condenser, cap etc.) The engine is running very well.

    The problem is there is no top end power. We can get the boat to about 3400 rpm and the risers are hot to the touch.

    I suspect there is a problem with the exhaust seals (flapper). I believe the overheat caused them to melt and dislodge from their place and clog the exhaust.

    I believe the seals that I have are these ones.

    Can these melt and fall into the Y-pipe or outdrive? The part where the bushing and shaft go through just looks like solid rubber and could easily let go.
    1989 Avanti 2955
    460 Cobra

    Not knowing the cobra well but risers (exhaust elbows) being hot to the touch sounds like a lack of water flow to/through them. Also a good idea to check the condition of the exhaust shutters after an overheat.
    Edmonds, WA
    "THE FIX" '93 2556
    Carbureted 383 Vortec-Bravo II
    The Rebuild Of My 2556
    My Misc. Projects


      You did not mention but I'm assuming you changed the impeller. First I would remove the hose coming from the transom assembly to the t-stat housing to verify the raw water pump is pushing a good flow of water. The flappers are rubber coated to keep them from clanking but they do fail and can restrict the exhaust. I would not operate the boat until thus is resolved or you will damage the engine as it's already being stressed if it loosing power.
      1997 Maxum 2400 SCR 5.7LX Bravo II



        Yes, the impeller has ben replaced and decent water flow has been verified. I don't believe there is any problems with water flow although initially that is what would be suspected.

        My thinking (along with other boaters via Google) is the flappers have failed because of the heat and the top end power is restricted because the exhaust is somewhat blocked. And the risers are getting hot also because of the restricted exhaust.

        I will be running this by my very capable mechanic. I will have him do a visual on the flappers. Just looking for some feedback here too.
        1989 Avanti 2955
        460 Cobra


          Raw water cooled ?

          Elbows have to be inspected, and flappers.

          Saltwater raw water boat ?

          5 years tops on the elbows.
          Be good, be happy, for tomorrow is promised to no man !

          1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

          '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

          Manalapan N.J


            Yes the flappers are just rubber and a bad overheat will melt them; they can drop down and wind up in the bottom of the exhaust housing.

            I had this happen once and removed the drive to fish them out with a coat hanger wire. When the impeller failed the first time did it lose any rubber vanes? If so did you check if they got lodged in the P/S cooler? Also check the thermo housing and make sure the ports feeding the exhaust are open. These neck down with corrosion and will restrict water flow. In salt water we replace risers and manifolds every 5/6 seasons and I've replaced that thermo housing 2x in 15 years....

            Is this a salt or fresh water boat?

            Makes a big difference in the maintenance you have to do for the cooling system.

            I would remove the thermo housing and thermostat and inspect; if not corroded re assemble with a new stat. You may want to remove the risers and inspect as well. They should not get hotter than about 145 or so...
            88 Four Winns 200 Horizon 4.3 OMC
            98 Jeep Grand Cherokee 4.0/Selectrac
            07 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

            Long Island Sound Region