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  • #31
    I would get out the pencil and pull the plugs again,
    feel the pistons to see if you can feel and slack in rod bearings.
    maybe compare the 2 cylinders to get a feel of them while at TDC while rolling the motor flywheel back and forth

    that is a bad knock,
    from the sound I do not think it is in the lower gear case

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    • #32
      Does it knock when increasing rpm while still in neutral?

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      • #33
        You can use a hard handle screwdriver like a stethoscope. Touch the screwdriver tip against the block, valve cover, etc. Then put your ear firmly against the end of the screwdriver handle and plug your other ear with your finger. It works good.

        I find a long Phillips tip driver works best..rather than a flat head.

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        • #34
          Originally posted by panasonic View Post
          You can use a hard handle screwdriver like a stethoscope. Touch the screwdriver tip against the block, valve cover, etc. Then put your ear firmly against the end of the screwdriver handle and plug your other ear with your finger. It works good.

          I find a long Phillips tip driver works best..rather than a flat head.
          Good tip panasonic.

          I also use a length of water hose to hear / locate where an engine noise is coming from.

          Can you replicate the noise in neutral?
          If it is one cylinder or the other, could the engine be run on one cylinder (plug wire removed) and then swap (plug that wire back then remove the other) and start on the other one cylinder again to determine if #1 or #2 is the noisy cylinder?
          **Please do not touch high voltage wires while you have a wet foot in the bilge. Shut the engine off between switching wires.

          * if it is a cylinder, 1 or 2 does not matter other than now you know for sure it is A cylinder and a decision is to be made...

          Did you inspect under the cowling that the linkage to the spark advance is connected and moving properly as the throttle is advanced?
          After it smooths out, do you feel like you are getting full power or is it lacking?
          Last edited by FabricGATOR; 05-17-2019, 01:02 PM.
          If its got teats or tires, you bound to have trouble with it....

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          • #35
            I did both the test for the lower end and pistons this morning. I took out the plugs, and with the engine in gear, I could easily turn the prop by hand. I was a smooth rotation without catches or skipping. The pencil in the cylinders produced the same results. Each piston seemed to glide the entire length of the stroke without catching. That was good news, I thought.

            A storm seemed to be gathering and the wind and chop in the harbor came up so the spark plug test will have to wait 'till tomorrow.

            Yes. It does knock when increasing rpm while in neutral.

            Can't avoid a wet foot in my bilge so I am very hesitant to screw around with hot wires. What I will try is starting the engine with one wire disconnected. If it starts, then I would figure that cylinder to be good. Then I'll stop the engine and test the second cylinder. Will that work? Anyone know of a good spark plug tester the tests while the plug is installed and connected?

            I believe the linkages are set properly, but to be honest, that's a guess. I'll check the service manual and see how to set up the spark advance. When I hit about half throttle, things smooth out and the engine pulls even and strong.

            Thanks for all the ideas and helpful information. I'm learning a lot.

            Bry

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            • #36
              The point of pulling the spark plug wires (one at a time) is to LESSEN the impact /noise IF a CONNECTING ROD BEARING IS TOAST.

              As 99Yam pointed out earlier, (but perhaps not quite understood), is the movement @ TDC. IF a bearing is bad you should be able to FEEL slight movement.
              IE, turn the engine(by hand) pushing a piston upwards. Once just past TDC, push down on the piston with a screwdriver and feel if the piston has some play BEFORE IT impacts the crankshaft.

              At the lower end of the stroke, the same method can be used. Your trying to feel for ANY PLAY between the piston/connecting rod changing directions and subsequently knocking(if the bearing is toast).

              Using the screw driver, etc, to hear the engine itself is to help pin point EXACTLY where the noise is emanating from...


              **Now that we have a good idea of your "noise", can you remove the cowl, crank up the motor and move about it, RECORDING (and again post) where about the noise is worst..


              Re the prop/ LU, I was looking for you to try and turn the engine over WITH the prop(spark plugs out), again, checking to see is there's a tooth missing.
              At this point, it sounds like a rod bearing, just trying to rule out the LU.
              Last edited by TownsendsFJR1300; 05-17-2019, 03:31 PM.
              Scott
              1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR

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              • #37
                Thanks Townsends. I got it. I'll try again tomorrow.

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                • #38
                  I meant to hold the pencil against the top of piston as it neared TDC but not quite there yet and then stop and turn the flywheel in the other direction., paying close attention on how far the flywheel moves before the piston moves back in the other direction

                  see if one piston takes more rotation of the flywheel before it starts moving back downward,
                  it it does then either the rod bearing on the crank or the wrist pin is failing and has slop

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                  • #39
                    Tried the pencil test again, this time using a screwdriver to try to “feel” for any sloppiness. I couldn't detect anything like you describe, but then I'm not a mechanic and really don't trust the results I'm getting. Also, I'm having to do this on the water. Not the most effective workbench :-) So once again, hanging off the end of the boat trying to get an accurate feel for the bearing isn't easy.

                    And sorry. I'm not fooling with high voltage wires with the motor running. But I did try my test: starting the warm engine with the spark plug wires alternatively disconnected. It started on each cylinder. That to me was bad news. Everything is pointing to something I can't fix. A bearing.
                    Bry

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                    • #40
                      FabricGATOR:

                      My service manual was written for mechanics: tells you what to do, not how. Anyway, not sure how to check the spark advance on the Yammie 15. And can't find a suitable answer on line. Least not yet. If you can point me in the right direction, that would be great and I would certainly appreciate it.

                      Bry

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                      • #41
                        Originally posted by yellowtail View Post
                        Tried the pencil test again, this time using a screwdriver to try to “feel” for any sloppiness. I couldn't detect anything like you describe, but then I'm not a mechanic and really don't trust the results I'm getting. Also, I'm having to do this on the water. Not the most effective workbench :-) So once again, hanging off the end of the boat trying to get an accurate feel for the bearing isn't easy.

                        And sorry. I'm not fooling with high voltage wires with the motor running. But I did try my test: starting the warm engine with the spark plug wires alternatively disconnected. It started on each cylinder. That to me was bad news. Everything is pointing to something I can't fix. A bearing.
                        Bry
                        Did the noise lessen any with the plug wire (either) disconnected? We don't care if it runs, we're trying to narrow down the potential damage/ possible bad bearing..

                        With one cylinder NOT firing, the noise should lessen, IF the cylinder has a loose bearing..
                        Scott
                        1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR

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                        • #42
                          Sorry. Let me do the test again and listen more carefully. I was so startled that the engine started, I wasn't paying attention.
                          Thank you... Bry

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                          • #43
                            Originally posted by yellowtail View Post
                            FabricGATOR:

                            My service manual was written for mechanics: tells you what to do, not how. Anyway, not sure how to check the spark advance on the Yammie 15. And can't find a suitable answer on line. Least not yet. If you can point me in the right direction, that would be great and I would certainly appreciate it.

                            Bry
                            I'm not familiar with your particular engine, rather the small bore outboard engines in general. I was just curious if there was an active linkage that connects the carburetor to the ignition 'plate' under the flywheel.
                            I seem to remember as a kid, running one with that linkage either intentionally or unintentionally disconnected and it making a funny noise and loss of power.
                            Perhaps this is all just a result of that linkage becoming disconnected.
                            That is also why I followed up to ask if it seems like you have the same power that you had previously before the noise happened.

                            Do you have a beach nearby? I would drag my skiff up the beach a little ways, backwards by the tilted engine, so I could stand on a solid ground for some of the 'looking over' inspection.

                            It is my greatest hope and wish that we help you find an easy and inexpensive 'fix' for this...
                            Send me a plane ticket out of Miami, I'd gladly come down and take a look. lol.

                            Any chance you have any buddies that have a similar engine down there?
                            Last edited by FabricGATOR; 05-18-2019, 05:27 PM.
                            If its got teats or tires, you bound to have trouble with it....

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                            • #44
                              Originally posted by TownsendsFJR1300 View Post

                              Did the noise lessen any with the plug wire (either) disconnected? We don't care if it runs, we're trying to narrow down the potential damage/ possible bad bearing..

                              With one cylinder NOT firing, the noise should lessen, IF the cylinder has a loose bearing..
                              Yes, a bad cylinder may make less (or none) noise with its ignition wire disconnected.

                              I would try to limit the amount of time that the engine is in that "horrible sounding clatter racket" speed, but would do it briefly for troubleshooting purposes.

                              So, disconnect a spark plug wire and start the engine on one cylinder. Increase the rpm to see if it can make the noise and then shut it off.
                              Reinstall that spark plug wire and repeat the test with the other spark plug wire disconnected. Shut is off as soon as the test is complete.

                              Nevermind looking for what I was saying before about the spark advance linkage, that was hundred year old technology, I think your engine is electronic spark advance.
                              Last edited by FabricGATOR; 05-18-2019, 05:25 PM.
                              If its got teats or tires, you bound to have trouble with it....

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                              • #45
                                Well just did the test again and things sound a lot different with the cowling off but the vibration is still there so the cowling reverberates the sound and is not rattling. (Boy. That would have been too easy!)

                                So same as before. Warm engine and pull-start on each cylinder with the opposing plug wire disconnected. Throttle up a little... I could NOT detect any difference in sound between the two cylinders.

                                As far as similar engines the harbor? Yeah. A ton of them. The local fishermen use Yamaha 15hp Enduros, exclusively. So that's why cruisers from the US buy Yamaha 15's to install on their inflatables. Parts are readily available.

                                Unfortunately, honest mechanics in my neck of the woods are harder to find. Any guy here with a 10mm wrench, a screwdriver and a can of carb cleaner will happily tell you he is a qualified mechanic and tear your engine down for you. Problem is, he'll sell it off for parts and swear to you he is waiting for a shipment of parts.

                                The only way I would use a mechanic here is if I bought my own parts and could stand over him and watch while he did the job. I think changing bearings is a little more complicated than that. Tell me if I'm wrong.

                                Bry

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