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Knocking or Hard Rattle In Outboard

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  • Knocking or Hard Rattle In Outboard

    Hey guys new on this site but it has tons of information. I recently bought a 89 daytona Sprint with a 150 merc outboard. It had not been in the water for about three years but ran great. After i bought it and took it out a couple times it started knocking. Now I noticed it when i was towing two sea doos because they had broken down. I dont think the gear oil has been changed in a while and have already bought the stuff to do so. Its not like a hard knock but almost a hard rattle. As im changing the oil does anyone have any good ideas to look at for my situation. It runs fine and all just sounds different. Thanks Again..


  • #2
    Re: Knocking or Hard Rattle In Outboard

    the first thing that i would think of is that one or more of your pistons is loose from the rod or the rod is bent or out of alignment. i think this is a pretty serious problem. the best way to check this is to take out all the plugs and to look in to the combustion chamber. turn the flywheel till the piston is in the full up position closest to you. then take a screw driver or somthing else stick it in ond poke the piston to see if it has any give or can rattle it without the flywheel moving. try this on all cylinders and post the results. im not a super experienced mechanic and this is just an idea.

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    • #3
      Re: Knocking or Hard Rattle In Outboard

      I will definately try that. Thats an easy test to try. The thing is, is that it only will knock when in drive. When in neutral i can rev the engine and there is no rattle at all. The motor sounds fine.

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      • #4
        Re: Knocking or Hard Rattle In Outboard

        does it only rattle while in gear?

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        • #5
          Re: Knocking or Hard Rattle In Outboard

          Yes it is only doing it while in gear.

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          • #6
            Re: Knocking or Hard Rattle In Outboard

            Its could be the lower end then....is it a rattle or a loud click that gets faster if you give it a little throttle?If it is the shifting mechanism may need adjustment, the clutch dog is worn,or the big nut behind the prop isn't at the right torque.

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            • #7
              Re: Knocking or Hard Rattle In Outboard

              on a second thought.....if you disconnect each spark plug wire one at a time while in gear does the sound go away?

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              • #8
                Re: Knocking or Hard Rattle In Outboard

                Its hard to explain its more like a soft knock or rythmic tic that goes along with the rpms. Higher rpms faster tick or knock or whatever the heck we call it. Im hoping since the oil hasnt been changed in there in like three years since its been sitting that maybe just needs some good fresh oil in the drive.

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                • #9
                  Re: Knocking or Hard Rattle In Outboard

                  You can take the shift cable off,and shift it by hand.If the sound disappears then the cable need adjustment,or replacement.

                  With the engine of turn the prop by hand,and get a feel for it.Put it in forward gear.It should click if you turn it clockwise,and lock if you turn it counter clockwise.Does it do it in forward and reverse gear?

                  I had this same thing happen to me.It turned out the big nut behind the prop wasn't torqued.Luckily I caught it before I junked my clutch dog.....

                  But I can be pointing you in the wrong direction.If you take the plug wires off while the engine is running (one at a time.) and the noise go's away say after you took off wire #2 then there is most likely a problem with that cylinder.

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                  • #10
                    Re: Knocking or Hard Rattle In Outboard

                    I have done the shifting by hand since i just recently replaced the shifter cable. it was knocking before the replacement. But it doesnt seem to make the noise when i have it out of the water and hooked up to the hose. But if it was a cylinder wouldnt it be knocking whether or not it was in gear?

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                    • #11
                      Re: Knocking or Hard Rattle In Outboard

                      Don't think so...I had a truck that wouldn't knock unless it was under load.I know its not a 2 stroke,but same concept....combustion engine that uses pistons.Thats why I suggested pulling wires at a time.With the wire off a particular cylinder it is not under as much strain and load.It might just stop knocking.Then you know its the power head.

                      But you have to pin point were the sound is coming from,and then go from there.

                      You can just pull the guts out of the lower end.Its easy...and inspect for chipped gears or dog clutch.


                      edit:after reading your last post over,Id go ahead and pull the lower unit,and check it out.

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                      • #12
                        Re: Knocking or Hard Rattle In Outboard

                        Drain the lube in the lower unit and examine it. If there's significant metal in it, or it is low or half water, you probably have a serious problem with the lower. If all is good, replace the lube, preferably with Merc High Performance (the green stuff)

                        Merc lowers often knock if they are out of adjustment, or the prop shaft bearing nut is loose. There is one combination of prop and LU that will knock like a bad rod at idle, and is perfectly normal. (It's a drag boat prop, not like on your boat) However, if it knocks at speed in time with the engine, I'd consider it major.

                        You can get to it and repair it while you have 1 or 2 parts to replace, or mess with it till you put the pinion out the side, then get in the market for a complete LU.

                        hope it helps
                        John

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                        • #13
                          Re: Knocking or Hard Rattle In Outboard

                          Thanks for all the help guys. Im going to replace the gear oil on saturday and test it on the water to see if it helps or not in the meantime looking for those things. I will let you know if anything comes up. Im hoping just its low on the gear oil but im doubting that is causing it..

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                          • #14
                            Re: Knocking or Hard Rattle In Outboard

                            ALRIGHT SO I WENT AHEAD AND CHANGED THE GEAR OIL. THERE WAS NO WATER IN IT WHICH IS GOOD. HOWEVER THERE WERE SOME SHAVINGS ON THE MAGNET. i DONT KNOW HOW BAD THIS IS SINCE I DONT KNOW THE LAST TIME THAT IT HAD BEEN CHANGED. ALSO I CHANGED THE GEAR OIL AND I FILLED IT UP WITH ONLY JUST UNDER A QUART. IS THIS A DIFFERENT TYPE OF DRIVE THEN NORMAL. THE OTHERS ONES IVE SEEN SAY BETWEEN TWO TO THREE QUARTS. WAS NOT ABLE TO TAKE IT OUT THIS WEEKEND BUT WANTED TO UPDATE YOU GUYS ON IT AND SEE IF YOU GUYS HAD ANY QUESTIONS OR COMMENTS. AS ALWAYS THANK YOU. MATT

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                            • #15
                              Re: Knocking or Hard Rattle In Outboard

                              Does anyone have any other ideas....

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