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The mysterious PWC

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  • The mysterious PWC

    I've been boating since 2011 and most of that has been bass boats and off shore boats. I just purchased my first wave runner last week. It is a 1996 Waverunner 760. It says 203 hours on the dash. The guy I got it from owned it for the last two years and said it ran perfectly aside from some sputtering on the low-end. He credited this to possibly a rich fuel mixture because he still added oil to the gas tank in case the oil pump ever failed. I test drove it and I did notice some sputtering on the bottom end RPM but once I got to the upper power band it ran perfectly over 50 miles an hour. I bought the ski and trailer for $800 and it looks in immaculate shape. I was curious about the compression and I know I should've done that first. I went to AutoZone and did their loaner tool program and got a compression tester.

    Front "0"
    Rear "59"


    I knew something couldn't be right. I went back to AutoZone and said the gauge could not be reading properly. I gave it back and headed to the local marina. The guy there said he doesn't really work on ket skis but he would do a free compression test anyway.

    Front "39"
    Rear "135"

    He said that the ski needed a new engine or a rebuild both of them quoted about $1500. Also told me that there is no way the ski could even possibly run let alone start up. He further said that a rebuild would need to be done ASAP.

    Here's the strange part. The ski fires right up hot or cold. I did the sea foam spray fog to clear the carbs and replaced the plugs. Ran it at the lake to clear out the white smoke.

    Sputtering is almost all gone and the top end I picked up 3mph.

    Could the guy at the marina just want money?


    Can those numbers be right?

    I'm confused.

    Thanks!!

    Joe

  • #2
    Numbers are unusual, but what isn't unusual is for one of the power valves to break off and go into the cylinder.
    I have literally just been through the same thing. If you catch it in time and the engine is still running fairly well...thrown it just a top end rebuild it needs. I'm half way through mine. Couple of hours to strip it and put back together same again. Just need a new power valve and oversized piston. Good for another 100 hours after that.

    Comment


    • #3
      Originally posted by QBhoy View Post
      Numbers are unusual, but what isn't unusual is for one of the power valves to break off and go into the cylinder.
      I have literally just been through the same thing. If you catch it in time and the engine is still running fairly well...thrown it just a top end rebuild it needs. I'm half way through mine. Couple of hours to strip it and put back together same again. Just need a new power valve and oversized piston. Good for another 100 hours after that.

      Man that's good news!! What would that rebuild include? Like pistons and rings and gaskets? Or valves also?

      Thanks!

      Comment


      • #4
        Or could his gauge be off as well OR could he just want a job out of me? The strange thing I swear it runs like a top.

        Comment


        • #5
          Originally posted by jnatale3 View Post


          Man that's good news!! What would that rebuild include? Like pistons and rings and gaskets? Or valves also?

          Thanks!
          Hi
          I wouldn't bet on this being the problem...just saying that these engines are famous for it.
          Mine was running superb first few times out...noticed a bit of stuttering top end, then eventually a slightly reduced top end...only a few mph, but knew it wasn't just right.
          Did a compression test...
          Back cylinder was 118 psi
          Front cylinder was 80psi, then 40 psi straight after !

          Stripped it all down...not as much a task as you think, these yamahas are well engineered. 2 hours had it stripped down to the pistons.
          Then once you have the 2 cylinders casings off...send them away to get new pistons sized. You will need new oversized pistons if the power valve had failed...or even just if you have covered 100 hours since last rebuild...I'm afraid it's just standard for a high revving 2 stroke.
          Once you have the pistons back and perhaps worst case, your cylinder units have been re coated and honed, stick it all back together again. Bingo.
          It's remarkably easy to do.
          If you want to find out quickly....take the power valve lids off and have a look, but I'd be fairly certain this is what's happened. To do this you may have to take the exhaust manifold off on the port side of engine.

          Comment


          • #6
            Ps...mine was running well to, even though it has this problem.
            Pretty sure that if I didn't know what it was like before, I would have kept on running it. Started fairly well and still did about 50mph....just that it did about 56-58 before and I knew it just wasn't quite right.
            If I had left it longer and kept on running....then you are talking a full rebuild and probably spending much more than mine and yours ski is worth !

            Comment


            • #7
              Originally posted by QBhoy View Post

              Hi
              I wouldn't bet on this being the problem...just saying that these engines are famous for it.
              Mine was running superb first few times out...noticed a bit of stuttering top end, then eventually a slightly reduced top end...only a few mph, but knew it wasn't just right.
              Did a compression test...
              Back cylinder was 118 psi
              Front cylinder was 80psi, then 40 psi straight after !

              Stripped it all down...not as much a task as you think, these yamahas are well engineered. 2 hours had it stripped down to the pistons.
              Then once you have the 2 cylinders casings off...send them away to get new pistons sized. You will need new oversized pistons if the power valve had failed...or even just if you have covered 100 hours since last rebuild...I'm afraid it's just standard for a high revving 2 stroke.
              Once you have the pistons back and perhaps worst case, your cylinder units have been re coated and honed, stick it all back together again. Bingo.
              It's remarkably easy to do.
              If you want to find out quickly....take the power valve lids off and have a look, but I'd be fairly certain this is what's happened. To do this you may have to take the exhaust manifold off on the port side of engine.


              Do you know of the link or the part number? Or can I get a standard kit like from Amazon?

              Comment


              • #8
                Originally posted by QBhoy View Post
                Ps...mine was running well to, even though it has this problem.
                Pretty sure that if I didn't know what it was like before, I would have kept on running it. Started fairly well and still did about 50mph....just that it did about 56-58 before and I knew it just wasn't quite right.
                If I had left it longer and kept on running....then you are talking a full rebuild and probably spending much more than mine and yours ski is worth !


                I hear ya! Looks like a winter project getting ready for the spring!!!

                Comment


                • #9
                  Hi
                  All I have done is stripped down the engine to the cylinder units and pistons.
                  Took cylinder units off and sent away to Yamaha specialist to get 're sized pistons and rings. They will now send right sized pistols, rings and a top end gasket set to rebuild. Also sending me a set of new power valves too.
                  You can buy all this stuff yourself much cheaper but I figured I should let the expert sort that out for me and I just do the spanner work.
                  Like I say....have a look at the power valves...id imagine that one of them will have broken and been let loose in a 7000rpm crushing machine !. If Not, it will just be because she has done 200 hours and either just due for her second routine top end rebuild....or perhaps even the first....its just a standard thing with these 2 strokes. But like I say...good news is you have a Yamaha and they are well engineered and designed to be taken to bits every 100 hours.

                  Comment


                  • #10
                    Originally posted by QBhoy View Post
                    Hi
                    All I have done is stripped down the engine to the cylinder units and pistons.
                    Took cylinder units off and sent away to Yamaha specialist to get 're sized pistons and rings. They will now send right sized pistols, rings and a top end gasket set to rebuild. Also sending me a set of new power valves too.
                    You can buy all this stuff yourself much cheaper but I figured I should let the expert sort that out for me and I just do the spanner work.
                    Like I say....have a look at the power valves...id imagine that one of them will have broken and been let loose in a 7000rpm crushing machine !. If Not, it will just be because she has done 200 hours and either just due for her second routine top end rebuild....or perhaps even the first....its just a standard thing with these 2 strokes. But like I say...good news is you have a Yamaha and they are well engineered and designed to be taken to bits every 100 hours.


                    Can I do all of this with the engine still in the hull?

                    Comment


                    • #11
                      Yeah.

                      Comment


                      • #12
                        Originally posted by QBhoy View Post
                        Yeah.


                        I took the cylinder head off today. The top of the low cylinder pressure piston has a lip folded back maybe 1/4 inch. It's the top of the piston that goes up and down on the exhaust side. The sleeve had about 4 tiny grooves that I could barely feel. The cyclinder also had a few droplets of water in it and the head gasket looked like crap. I sanded the wall with a very fine Emory cloth and oiled the cylinder. I also replaced the gasket and put it back together. I am still buying a rebuild kit but I wanted to see if this helped with compression and it did a littl. The rings seemed to look ok from what I could see. Isn't the water droplets normal if the engine was shut off while hose on? Should I replace both sleeves? Or just the two pistons?

                        Comment


                        • #13
                          Hello, first post on the forum

                          I owned and rebuilt a 98 Wave Venture 760, chances are it is the exact same engine you have. I noticed someone mentioned power valves, this motor should not have power valves. On the other hand these motors are super simple to work on, parts are fairly inexpensive compared to other skis I have seen/worked on and generally they are bullet proof! I find it odd the motor has issues and at the same time the guy added oil to the gas "in case the oil lines popped off" while that is a common problem it still strikes me as odd in the sense that maybe the oil lines did pop off on him at one time and it began to have a light oil loss seize.

                          There are for sure many approaches you could take, grab a top end kit from SBT or take your cylinders to get them bored and honed to accept new oversized pistons, honed and have tolerances checked and go with new stock pistons and rings. Have you checked the tolerances of your cylinders with a caliper to see if they are within spec?

                          Water droplets on cylinder could be from the disassembly, could also be a faulty gasket or wash in the face of the cylinder, you definitely don't want water getting into your cylinders while you are running. Water can't compress and enough of it will jack a motor up quick and in a hurry.

                          One other thing you may want to check into while you have things apart is a carb rebuild, again fairly straight forward and kit's should be readily available. Again SBT is a good place for warranty parts though they aren't always of the best quality you can buy, I have used one of there cranks, gaskets and other misc parts with no fail in the past.

                          Some pictures would help a lot!


                          Comment


                          • #14
                            Originally posted by override View Post
                            Hello, first post on the forum

                            I owned and rebuilt a 98 Wave Venture 760, chances are it is the exact same engine you have. I noticed someone mentioned power valves, this motor should not have power valves. On the other hand these motors are super simple to work on, parts are fairly inexpensive compared to other skis I have seen/worked on and generally they are bullet proof! I find it odd the motor has issues and at the same time the guy added oil to the gas "in case the oil lines popped off" while that is a common problem it still strikes me as odd in the sense that maybe the oil lines did pop off on him at one time and it began to have a light oil loss seize.

                            There are for sure many approaches you could take, grab a top end kit from SBT or take your cylinders to get them bored and honed to accept new oversized pistons, honed and have tolerances checked and go with new stock pistons and rings. Have you checked the tolerances of your cylinders with a caliper to see if they are within spec?

                            Water droplets on cylinder could be from the disassembly, could also be a faulty gasket or wash in the face of the cylinder, you definitely don't want water getting into your cylinders while you are running. Water can't compress and enough of it will jack a motor up quick and in a hurry.

                            One other thing you may want to check into while you have things apart is a carb rebuild, again fairly straight forward and kit's should be readily available. Again SBT is a good place for warranty parts though they aren't always of the best quality you can buy, I have used one of there cranks, gaskets and other misc parts with no fail in the past.

                            Some pictures would help a lot!



                            Just got done today. .50 bored over at the machine shop. Installed a top end kit and WOW!!! What a difference. I rebuilt the carbs too and I can feel it wake up from a dead stand still. Taking it easy for a break in but the whole deal (not including machine shop time) was about 9 hours.

                            Comment


                            • #15
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                              Originally posted by jnatale3 View Post



                              Just got done today. .50 bored over at the machine shop. Installed a top end kit and WOW!!! What a difference. I rebuilt the carbs too and I can feel it wake up from a dead stand still. Taking it easy for a break in but the whole deal (not including machine shop time) was about 9 hours.

                              Hell Yeah! Glad you got her back to running again Have fun! From the 1st to the 4th is dang quick on getting it all done and back together.

                              I would recommend looking up a guide to follow for proper break-in, mix oil slightly heavier (or add some oil if you are still using oil pump) and take it easy for the first full tank varying throttle and giving it cool down breaks so your rings seat properly. After a full tank give it some hell but still taking it easy, third tank she will be good to go!

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