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  1. #1
    sdlvx's Avatar
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    98 Pro 785 without a thermostat popoff valve

    Hello everyone. I'm recently putting back together my 98 pro 785 after a four point seizure. The seizure came late last season when I was out (it was cold, in the 60s probably) and the water was cold.

    Anyways, I was looking through the Pro 785 service manual, and it said the 1998 model does not have a thermostat pop off valve. I'm thinking that this is why the rear cylinder seized.

    I was wondering if there's a reason why there's not a thermostat valve in my boat, and if it would be a good idea to put one in there. I have a spare one laying around from a 1995 SL 750 engine that has barely any hours on it, we bought it for parts for 500 because a lot of the SL 750 parts fit the Pro.

    Thanks for the help


  2. #2
    casey67's Avatar
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    I think they're train of thought was, the Pro is a race ski-races are held in warm temps.

    It will depend of your riding conditions.
    Letting it warm up , idling ,should give you even engine temps.

    Cold temps=leaner conditions.... lean conditions = higher piston temps

  3. #3
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    because of the triple pipes and water routing you don't have the same pump pressure acting on the pop off valve.in the summer the motor will run at t-stat temps basically.

  4. #4
    sdlvx's Avatar
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    Thanks for the help. I think I'm going to go for it. I'm riding in SE Wisconsin and I'm usually out really early and really late in the season. If I start seeing the temp light in the summer I'll just take the t stat off. Now to figure out why my rear exhaust manifold water jacket is leaking. It's been one thing after another and I've been dropping so many hours on this ski.

    I'll give you guys an update on how it works if I ever make it out. I missed these few nice days we had up here and the weather is going to shit again soon.

  5. #5
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by sdlvx View Post
    I'm thinking that this is why the rear cylinder seized.
    Quote Originally Posted by sdlvx View Post
    Now to figure out why my rear exhaust manifold water jacket is leaking.
    Is the cooling system all stock, and how long has the rear exhaust manifold jacket been leaking?

    If everything is bone stock, then the water for the rear cylinder is fed through the rear exhaust manifold. Couldn't a leak theoretically rob pressure and volume of water from said cylinder?

  6. #6

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    i ride in cold water here in il. and have had a TON of pro's and never had them stick a piston you must have something else going on
    first you need to fix that leak and do you know if your running Factory Polaris pistons or Wiseco's ???
    there is no need for a tstat

  7. #7
    sdlvx's Avatar
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    Alright, so I found the cause of the leak. The rubber grommet between the flanges of the rear and center exhaust manifold was caught and it wasn't letting the bottom of the manifold seal up properly as the grommet was getting caught on the flange. I was torquing down on the grommet and flanges and not the manifold and exhaust port.

    I did put the thermostat on and I think I'm going to take it out. It seems like it's malfunctioning because of the change of pressure from the pipes. I think it's sticking closed if you let it idle too long. That and the center cylinder is getting warmer than the rear and front.

    I haven't made any major changes to the cooling system other than the t stat. I had to replace the hoses on the pipes last year because some of them rubbed through and were leaking. The rear manifold was only leaking after I put it back together because I screwed it up.

    I'm running with wisecos, la sleeve doesn't offer the pro-x pistons anymore and my dad is a dealer so we get them cheap. When I seized the rear cylinder, it was late november at night when it was cold. I did throw some Polaris oil in the oil tank when I was running Klotz, I'm not sure why that'd only seize one piston and not all 3. Not to mention they were both synthetic and there was not a lot of klotz left in the tank.

    The rear cylinder has been problems before and we've already had to throw new pistons in it thanks to someone putting a wrist pin clip in the wrong way from the factory or from the dealer we bought it from. But we did put pro-x pistons in there.

    I'm still at a loss for what caused the seizure and to be honest I'm pretty worried about heading out again. I just spent about $400 because the rear cylinder needed to be sleeved because it seized pretty deep. The only thing I can think of was that the piston warmed up faster than the cylinder, but when I seized it I went through an entire tank of gas and was beating on it for a full tank.

    Earlier that day I started to lose a ton of top end, and I was only revving out at about 7k RPM and making it into the lower 50s, but everywhere else in the power band was fine.

    I put wisecos in all three cylinders now, the other two cylinders could have used them I think.

  8. #8

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    if you run wiseco's you should have the cylinders redone to get the right clearance, the wiseco's need atleast 6 thousenths clearance and tend to stick in stock cylinders unless you carefully warm up the engine before hammering the throttle .I like to warm them up before i get it into the water

  9. #9

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    Pto cylinder..... Usually water intrusion, or crank phase problem.

    Don't run the engine until you figure out exactly what caused it. Very likely going to happen again.

    Could be many things, but this is where diagnosing comes into play.

  10. #10
    sdlvx's Avatar
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    Quote Originally Posted by 785 lugs View Post
    if you run wiseco's you should have the cylinders redone to get the right clearance, the wiseco's need atleast 6 thousenths clearance and tend to stick in stock cylinders unless you carefully warm up the engine before hammering the throttle .I like to warm them up before i get it into the water
    That's what's been bothering me a little. We only left about 4 thousands on each cylinder (maybe a half a thousandths extra). I know I'm going to have to baby it to warm it up and that's a lot of my reasoning behind putting the t-stat in. If the t stat is going to keep the cylinders warmer it should help with the wisecos expanding faster since they're forged. We honed the stock ones out which should give a little extra room, but since they're stock I don't think we could get the extra .002 out of it.

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