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Engine misfires

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  • Engine misfires

    I am struggling to resolve an engine misfiring issue. The boat runs fine when just starting up and idling. At cruise it runs fine at 3600 rpm for about 10 mins and then it starts to miss and the tach bounces 2 to 3 hundred rpm. At this point it will not rev above 3200 rpm at full throttle. I have change the distributor cap, rotor, electronic ignition, fuel pump, and coil. Carb was changed 3 years ago with a new Holley marine direct replacement of original. I have also drained the gas tanks twice and it has fresh non ethanol fuel. Engine has 807 hours and does not smoke or burn oil.

    I’m out of ideas what to do next other than rebuild a 3 year old carb. Anyone have an suggestions?

    Thanks!

    Pat


  • #2
    Look at your fuel lines for a possible area of vapor lock.
    1988 26' cuddy
    1975 20' open
    16' Sandpiper
    14' Certified Fiberglass

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    • #3
      Try burning out of a remote fuel tank.Like a 5 gal outboard tank. Right to the fuel pump direct. Then you will see if it is tank/ fuel line related
      '78 20ft Cuddy
      Mercruiser 350 FWC preVortec w/Carb

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      • #4
        Open hatch at night engine running. Look for arc'ing between plug wires or eng.

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        • #5
          What should I be looking for with my fuel lines for a possible vapor lock issue?

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          • #6
            I have rubber fuel lines, after too many years I noticed symptoms similar to yours. Found the lines starting to perish, especially around the selector valves at the transom. I wasn't leaking fuel yet but air was getting in. I'd start there and proceed to the fuel pump.
            Enjoy,
            Tony

            1981 20 ft cuddy
            4.3L Chev V-6

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            • #7
              Any luck so far?

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              • #8
                Changing out the final fuel line today. I did find a compressed fuel line and almost completely clogged vent on the port side tank. The tank itself looks like it is compressed slightly also. The starboard vent was restricted also but not as bad as the port side. We’ll see what happens when I get it a test ride. Fingers crossed!

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                • #9
                  Did a test run today and one of the main issues is still happening. For the first 10 or so minutes engine revs up to 3800rpm ( probably goes to 4100 just didn’t push it today) runs fine and no engine miss. After10 mins the
                  engine struggles to reach 3200-3300. The throttle is buried but it will not exceed 3200-3300 rpm. The good news is there is no rev jumping and the engine does not miss that I can tell. I’m down to the carb as a final area to try. Once the engine gets warmed up, the secondaries of the carb don’t seem to open and the engine bogs down and labors. Anymore ideas guys?

                  Thanks for all the replys.

                  Pat

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                  • #10
                    32-33 is where the secondaries should come in. Mess with the linkage before buying a new carb. If they open when cold, the question is, why not when hot ? Can you see them when running. Move them by hand ?
                    1988 26' cuddy
                    1975 20' open
                    16' Sandpiper
                    14' Certified Fiberglass

                    Comment


                    • #11
                      How long does the engine need to rest before it runs normally again? When the problem occurs, does the engine run normally below 3000rpm, at cruise, idle, etc.?

                      Have you checked the fuel pressure at the carburetor (under load) or tried a remote fuel tank like Pedlyr suggested?

                      Have you checked the voltage at the ignition module? It should be very close to the voltage at the alternator.
                      Steve
                      1998 260 Mackinaw Sea Maxx 5.7 HO TBI 12' Klamath w/ 20hp Tohatsu EFI

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                      • #12
                        Quick Fix….I’m with you. Why would the secondaries not open after 10 mins of running. Linkage bound up after warming up? Does the electric choke have anything to do with secondaries? The next test run it’ll see if I can open the secondaries while running.

                        Tuner…I don’t have the answer to your first question but on the test ride today the engine ran well up to 3000 rpm when the problem occurred.

                        No to the fuel pressure test. Don’t know how to measure that. And no to the 5 gal remote fuel test.

                        And no the voltage testing of the ignition module and alternator Not sure how to do that. It has a new pointless ignition and new coil with the correct resistor value.

                        Most frustrating problem I’ve ever had to deal with on any marine engine.

                        thanks guys for you feedback.

                        Pat

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                        • #13
                          Did it do this before the magnetic ignition ? Check your installation instructions about a resistor in a magnetic ignition.
                          1988 26' cuddy
                          1975 20' open
                          16' Sandpiper
                          14' Certified Fiberglass

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                          • #14
                            Quick Fix….when I purchased the boat 7 years ago it already had the magnetic ignition…..a Pertronix. I just bought the same unit for replacement. I also bought a Pertronix coil with the correct resistor.

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                            • #15
                              So no inline resistor. Back to the secondaries.
                              1988 26' cuddy
                              1975 20' open
                              16' Sandpiper
                              14' Certified Fiberglass

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