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SL 780 Piston #1 problems

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  • SL 780 Piston #1 problems

    Hi,
    I've seen the Polaris tech tip about burning a hole in the #1 piston on Fuji motors and adding a second fuel pump. My question is whether there are any other known issues that may cause seizure or damage to the #1 piston on a 1996 SL 780 (oil injection system , ignition etc etc). After 18 months of trouble free riding my SL 780 has blown the #1 piston twice in less than 10 hours of riding and in each case the piston has been broken apart. On the first occassion the top of the piston had big chunks of metal missing and screwed rings. On the second occassion the skirt on one side was completely gone and the skirt on the other side was cracked with rings that were once again stuffed. I have read that the diaphram in the fuel pump can weaken with time and have problems delivering fuel to the #1 carb at WOT so I will at least replace the OEM fuel pump and probably add the second one as recommended in the tech tip. I don't want to bolt it back together and have it go again so any ideas as to other places to look would be much appreciated. Another quick question .. what do the numbers on top of the head mean ?? I have 2 heads that are stamped with a 1 and one head that has a 2 on it ??.

  • #2
    While you certianly may have a carb. pump problem, the problem you refer to was only applicable to the 650 and 750's - they fixed the problem with a higher volume pump on the 780.

    Without pictures it's going to be impossible to diagnose your previous failures, however it's not out of the question, from your brief description, that you had a detonation problem first, and too much piston to cylinder clearnace the second time.

    Who did the rebuilds and what were their diagnosis?

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    • #3
      Thanks for the prompt reply. The rebuild was done by a local repair shop. They checked the bore , honed it and declared it fine to re-use after replacing the piston ,rings , bearings , gaskets etc. They also checked the carb to make sure it wasn't running lean and did a ride test. When I asked if they could identify the cause they shrugged their shoulders and said "dunno .. could be a number of things". I am at a disadvantage .. on both occassions that the piston blew the ski was on loan and I don't have any first hand knowledge of how it was performing before the failures. This could be totally irrelevant but both times the "borrowers" said that the ski was performing normally when it was put to bed but the problem showed up as soon as they put it in the water the next day when it was cold.

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      • #4
        Well, a dealer response like that is not what *I* would have paid to hear...unfortunately you are probably never going to know exactly what happened at this point.

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        • #5
          Was it ridden doubles when it seized? If so, the added load on a lean set up can lead to seizure? It might not seize with one rider if you are dialed in on the edge but as soon as there are 2 people, a seizure can rear its ugly head. I agree with what Bryan said. I would check the bore and make sure to have the correct piston to cylinder clearance and ring gap. It sounds like you had too much clearance and the piston was slapping around until it broke.

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