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  1. #1
    JohnCiccarelli's Avatar
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    94 GTX with No compression in Rear Cylinder. How can it run 40 MPH?

    I just got this Ski from a neighbor. I got it started and it runs, but not well. It is sluggish at low speed, but will top out at 40 MPH. My question is how does it run this well? If there is no compression in the rear cylinder, how can it go this fast?


  2. #2
    mibunkerking's Avatar
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    It is piggy backing off of the good cylinder.

  3. #3

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    Even with really low compression (unless literally has a hole in it) that cylinder is probably giving out 5-10hp and assuming the other is up to snuff it will be at its full 35hp (I think that's a 70hp motor?).. Means you have 40hp anyway maybe more. Add to that, the higher the RPM, the more compression a nearly dead piston will provide so it may be close to 1/2 output at max RPM but, as you said, a dog at lower RPM's

    Greg

  4. #4
    JohnCiccarelli's Avatar
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    I just pulled the head and there is a 1/2 inch hole in the top of the piston near the rings. I am again amazed at how well this thing ran.

    The next question. Can I possibly get away with just replacing the rear piston? The replacement will need to be .5mm over. This thing is old and I hate to spend too much to repair.

  5. #5

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    Not a Seadoo expert by any means, but.. What ever metal is missing from that piston will mostly be in the crank and bearings.. Had it been shut down as soon as it blew and if the hole was a break and not a melt out, maybe. Since it's been run, doesn't mater, expect those bearings to be toast so you'd need a piston set and a crank not to mention an answer to what did it.

    Greg

  6. #6
    JohnCiccarelli's Avatar
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    I do plan to tear it down and inspect everything. I have had several others apart with similar problems and have never had to replace a crank.

    My real question is, can you run a stock piston in the front and an aftermarket .5mm over piston in the rear with no problems? The front compression is 160.

    Any opinions?

  7. #7
    KrunchovXPL-GTX-RX's Avatar
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    You have a good chance on the crank. Like you, I have replaced a number of busted/melted pistons and have yet to find anything in the crank.

    I do suggest that you split the cases and spray the crank out (a lot) with fogging oil (my preference, but any lube would do).

    As to the pistons. It may well do just fine with one oversized piston (it is not like an RValve engine could have an MORE vibration to it). However, this flys in the face of conventional practices and would give you an inbalance between the two cylinders.

    All joking aside, I would suggest you not try to do this.

    An option would be to replace (or re-sleeve) that rear jug so you could run a standard size piston on the rear.

    Keep in mind that while it has been running on that dead rear cylinder it was stressing the hell out of the front. It may not be a problem, but you would surely hate to find out that it IS a problem ten or fifteen hours from now.

    Just a thought...

  8. #8
    JohnCiccarelli's Avatar
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    The difference is like $100 for the Piston, Rings, Bearing, and Cylinder Boring.

    I guess that you are right. I hate putting $250 in a Ski that is only worth about $770. By Spring It will be more like $600.

  9. #9
    Work Harder, Millions on Wellfare Depend On Us! ultramaxracing05's Avatar
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    you got lucky, i ran my seadoo with low compression and ended up blowing it up piston was destroyed in million pieces. it started smoking like crazy on take offs and i was pulling tubers all the sudden it just quit,

  10. #10
    KrunchovXPL-GTX-RX's Avatar
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    Quote Originally Posted by JohnCiccarelli View Post
    The difference is like $100 for the Piston, Rings, Bearing, and Cylinder Boring.

    I guess that you are right. I hate putting $250 in a Ski that is only worth about $770. By Spring It will be more like $600.
    You might be surprised what you could get for that thing.

    In good running order with some TLC and spit and polish. In the time between May and early July you can get a lot more than you think. I know of one guy who makes a killing on boats like that.

    You just have to have it ready to go (running, paperwork, ect) and put the word out at the right time.

    I think you could get 1800 to 2 grand for it (might need a trailer, though).

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