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First stumbling, now can't go past 3000 RPM - 350 MAG MPI

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    First stumbling, now can't go past 3000 RPM - 350 MAG MPI

    Got a strange series of events, maybe anyone can guess what I should check.

    1. Boat runs fine from idle to on plane up to 30MPH except between 2000-3000 RPMs it would stumble, once it was over that hump ran like a champ
    2. One week later, same behavior ignore it and go to get gas. Kid pumps in 9 gallons and said "its full" but I have an 85 gallon tank and it was on E. Scratch my head and turn on the boat, says 1/4 tank so I ask him to try again, this time he was able to get it filled all the way up, he said it wouldn't take anymore before for some reason, anyway get its fill up, and as a matter of fact dumped 1/4 - 1/2 galllon out the over flow when I started it right into the water out the side. Ignore it and put it in the slip and go home.
    3. Today with full tank of gas we head out of the harbor at no wake speeds, then once out hit the throttle no matter even if I go up to wide open throttle (WOT) it never gets past 3000 RPMs and never gets on plane. Defeated we just crusie the ocean at 10MPH for an hour and come back.

    Oil pressure, temp, etc all spot on, no errors out the vessel view mobile for the engine.

    Purchased an entire tune up kit, plugs, wires, cap, rotor, and fuel filter for next weekend, motor has 180 Hours on it, Mercruiser 350 MAG MPI. Just recently put on new idle air controller valve, water pressure sensor and an oil change.

    What do you guys thinks I should check, it feels like a fuel related issue.

    Fuel delivery issue, I’d check the fuel fill hose for a possible kink because of the filling issue and the fuel vent is likely plugged. The vent fitting on the exterior can become corroded restricting the breathability of the tank.
    Edmonds, WA
    "THE FIX"
    '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
    (.030 over-Vortec top end-part closed cooled)
    The rebuild of my 2556
    Misc. projects thread


      Not sure if you have the same problem but iceman was plagued by a similar issue for a year, see thread link below for his findings.
      1997 Maxum 2400 SCR 5.7LX Bravo II



        Start with the easy things. Make sure all your injector connectors are properly seated. Pop your distributor off and ensure it's clean. I had the same engine in my last boat and once in a while the injectors would get dirty and it would start to stumble and run rough like yours -- I found that a 1 gallon can of fuel with a full can of Seafoam in it, connected to your fuel pump and then run through the engine for 10-15 mins at 2500RPMs several times while letting the engine sit for 15-30 minutes between flushes really helped. All of these things are free/cheap and work you through the process of elimination.

        After that, you can then start digging into the more complicated items such as fuel pump, valves, internal sensors etc...
        1999 Bayliner 3388
        Twin Cummins 4BTA
        Fisherman, Cruiser, Boaticus-enthusiasticus-maximus
        Member Royal Victoria Yacht Club


          Got it all fixed. The tune up solved it. Very important point, use real OEM Mercury Quicksilver Cap and Rotor. Never mix and match brands as I found out. 2 weeks of scratching my head why it died in the ocean, towed in, and wouldn't restart until I took off the Duralast flat distributor cap and put on an OEM one with Quicksilver rotor, runs fine now. Thanks for all the help.

          List of things done:
          Distributor cap, rotor, spark plugs, wire set, fuel water filter, inline fuel filter, new fuel tank vent, IAC valve, and IAC filter under the flame arrestor.