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OK to run engine with one weak cyllinder?

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  • OK to run engine with one weak cyllinder?

    I have a '98 STX 1100. This is the story as it unfolded; Please bear with me if it's longish, I I don't want to omit any detail : I bought it with a blown #1 piston [that's the front one] at 64 hours. The PWC is mint otherwise. I put in an after-market sleeve, a new OEM standard piston, and new OEM rings on all pistons and new OEM gaskets all around. I also tore open the CV carbs, looking for why the engine had broken in the first place. I found nothing. I made sure the oil injection was operating, and bled the line. It's still in operation for now.

    The first mishap happened less than 5 hours after the rebuild, the first time I went WOT. The cooling hose came off the engine [my fault, 100%. I figured the clamp was tight enough. Now I know better] and it overheated pretty badly. After we put the hose back in, and drained the swamped hull, it seemed to run perfectly well. But I noticed it took a few more tries than usual to start it the next time I went out.

    The second mishap took place at about 90 hours: A friend capsized the Jet-Ski, and didn't know how to right it back up. The engine completely filled with salt-water, which was removed the same day. I managed to start it again by pouring pre-mixed gas straight into the carbs until I could tell gas was coming out of the spark plug holes. I ran it for 15 minutes on the rinse kit, and took it to open water 3 days later for an hour, just to make sure all the salt water was indeed gone. The engine ran normally, as it had since the rebuild.

    At 100 hours, for no particular reason, I decided to take out the plugs, for a little cleaning. That's when I realized that cylinder #1 [which was the rebuilt one] had no compression pressure to speak of! I didn't have a gage, but I can tell the difference between the other two cylinders, which make a powerful "PSCHTTTTT" sound when I plug the hole with my thumb and cylinder #1 which didn't make any sound with the engine cold. Please note I could see the piston moving.

    Since the engine had run perfectly the last time out, I decided to start it, to see what the hell was going on. [For all I knew, that cylinder was dead since the overheat event, so I had to find out if it sounded different]. I could hear a slight knock, which lasted 15 seconds or so. It wasn't very bad, so I let it run. I put the Jet-Ski in the water, and sure enough, it ran like it always did: 50 mph top speed, no weird noises, no apparent problems. I took the plugs out, and found *some* compression in cylinder #1, with the engine warm. I'll define *some* as less than 50% of either of the other 2 cylinders, not having a gage and all. [Incidentally, it's not that I don't want to get one, but I'm in the caribbean, and it's hard to get stuff from the 'States. I'm working on it]

    I have notice a few more times that light knock. It happens specially when restarting the engine after a 10-20 minute pause, and it goes away in 15 or so seconds. I don't know what causes it, but it's definitely piston or con-rod related.

    After all this background info, my question, finally: It seems to me that I can keep on running the engine. I understand that it's got a weak cylinder, but if it gives me enough performance as is, must I rebuild it now? I would rather do it in a few months. On the other hand, I don't want it to throw a rod or anything like that... So what's the opinion out there, can I leave well-enough alone, or should I tear it open without thinking twice, and thank my lucky star she's still running?

    Thank you much for any replies.

    [ December 20, 2002, 02:54 PM: Message edited by: gigi ]
    No more Jet-Ski's for this boy...

  • #2
    You need to stop running it and fix it, or you do risk throwing a rod and causing very expensive component damage.

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    • #3
      Thanks for the quick reply, Bryan. I was afraid someone was going to say that. It is reasonnable...

      Any ideas what might cause the knock? More to the point, is it possible for me to diagnose that myself, without special tools? I do have the shop manual, and I do know which way things should move and which way they shouldn't inside an engine. Still...

      What about the compression that comes back [well, partly anyway] with the engine warm? Spraying WD-40 in the cylinder sends it back up quite a bit. That points to rings, unless I'm mistaken.

      Dargon it, I guess I've got some work ahead of me! I was hoping to not have to pull that engine for another 150 hours! Haha.
      No more Jet-Ski's for this boy...

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      • #4
        I can't speculate on what's wrong without you diassembling it for inspection, but it's definitely partially due to ring-to-cylinder seal, yes.

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        • #5
          Cool!

          Thanks a lot, Bryan. Great articles also in the technical section [FAQ's]

          I'm going to bite the bullet, and tear it apart.

          Hopefully, (1) set of rings and gaskets will be all it takes. If I need to change anything in the lower end, that's really going to blow.
          No more Jet-Ski's for this boy...

          Comment


          • #6
            For the benefit of other people who might read this posting, let me say that while I was waiting for the parts to come in from the 'States, we ran the Jet-Ski 3-4 times. Of course, we ran it until the engine went bust. It was running fine, and then dropped to 2 cylinders. I wasn't riding it when it happened, but I found out the next time we took it out. A piece of the piston had shattered, and severely gouged the cylinder head. Pictures can be found here:

            http://www.sbtontheweb.com/cgi-bin/u...=000434#000000

            The moral of the story is: Listen to Bryan, and don't expect problems to go away, they never do, they only get worse.

            I consider we were lucky, all in all... But I do hope I don't have to do another rebuild for at least 3 years! I've junked the oil injector pump before I had even opened the engine! I've also installed a primer kit, with which I am extremely happy :D

            Gigi

            [ December 20, 2002, 02:52 PM: Message edited by: gigi ]
            No more Jet-Ski's for this boy...

            Comment


            • #7
              Although we are sorry to hear of your loss, we very much appreciate the update on your situation. Updates to problems are invaluable to a good Tech Forum.
              SBT Tech Support is here to help with your problems.
              We try to answer each question quickly and accurately.
              Please do not use Private Messaging for Tech Support, use the forums.

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