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  1. #1

    '95 780 SLX pre-season inspection :(

    So I put my ski away last year knowing it needed some help: weak fuel pump, PTO cylinder cap needs replacing, figure out where that odd knocking sound is coming from. Funny thing is, it started off the year supposedly being rebuilt(crank on up!)...but that's a whole other story I'm not going to get into.

    I took the heads off today. Inside I find the center piston looks like it was next door to being holed. I thought that was interesting because if memory serves correct, the spark plugs always indicated a rich mixture. I also find all 3 pistons seem pretty loose inside their cylidners at the top of the stroke, even worse at the bottom. UGH! I have a used cylinder & piston from ebay I figured I'd use to replace the one that was knocking. It's much tighter than the pistons currently in the engine. I haven't pulled the cylinders off yet to see what kind of condition the crank is in - hopefully it's ok. Hopefully the near-holing experience with the center piston was caused by a weak fuel pump.

    On all 3 pistions, I found the markings ".075". Can I have the cylidners bored over again or do I need to re-sleeve? Where do I get new pistons / sleeves? I saw a few pass by on ebay but thought, "Nawh - I don't need to go buying more spare parts." Dooh! What would you do at this point if it was your ski?

    Sorry for the rant...Just really bummed right now after I thought I only needed one new jug/piston but found out I really need 3. Not sure I want to put the money into it this year but I have kids itching to go riding!

    Thanks

    Dan
    Last edited by vanlohd; 05-15-2007 at 05:50 PM. Reason: corrected year of ski from '96 to '95


  2. #2
    ph2ocraft's Avatar
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    Hi Dan, WELCOME to the Green Hulk Forums!!
    If the crank is O.K., why replace all the pistons and cylinders?
    Take a stock or slightly larger cylinder and get it bored oversize. It's not the correct way to make the repair BUT it will work until you can gather up some parts and decide what you're going to do.
    You have a photo of the damaged piston and have you checked your crank index yet?

  3. #3
    Moderator beerdart's Avatar
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    Dan Welcome to the Hulk!!!
    Are you sure of the markings? Most are marked in mm's like W7A for 750 or .50 for .50mm over.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Greenhulk! The markings do sound amiss, most Polaris are .25 & .50 oversized. Forget the sleeving, way too much $$$. Send Atsaron a PM or reply in his thread he just posted. He has 780 cylinders sitting on his shelf. Also you can try PM John Zigler. He has alot of good used parts fairly priced. Get some pics if you can and post them please. Oh, and who did this crank-up rebuild?

  5. #5
    Hey guys, thanks for the welcome!

    ph20: I suspect the crank is ok. I'm going to pull all of the cylinders off so hopefully I can inpsect it closer. There is a scribe mark on the counter weights that should show if the indexing is OK. Once I have the cylinders off I plan on taking them to a local machine shop, along with the pistons, to get inspected.

    I hope to have pictures of the piston soon. I don't believe there was any damage to the head but I'll double checked.

    The markings on the 3 pistons are: 78A 0.75 stamped on the top of the piston. Any ideas what that means?

    Polaris Indy Specialty in Shakopee, MN rebuilt the motor. Unfortunatly I've questioned the rebuild since having the motor back. The heads & head gaskets were installed backwards by them and the head bolts were practically loose.
    Edit: For anyone reading this, I want to make sure it's clear: I'm not blaming them for my problems. At this point it's looking like I either made a mistake somewhere or just had a weak fuel pump. Dan was a friendly guy to deal with and did seem knowledgable when I droped the motor off.

    The center piston was OK through 95% of the seaon. I didn't figure out the head gaskets were bad until I had to replace one due to a imperfection that caused a leak in the PTO cylinder. I pulled all of the heads & replaced all 3. When I did this that I noticed how loose the pistons seemed. The center piston was OK at this point.

    I've PMd atsaron. Sounds like I may be getting some cylidners - even if they're just spares to keep this thing going for a while

    Thanks guys!
    Last edited by vanlohd; 05-13-2007 at 08:54 PM.

  6. #6
    One more quick though: After replacing the head gaskets towards the end of the year I was riding across the lake at WOT. In typical fashion the ski died on me. I believe this was happening due to a weak fuel pump. Anyways, when it died this particular time the engine would not turn over. I could hear the starter buzzing like it was trying to turn but no cranking. I let it sit for a minute & she fired right up. Ran fine for the rest of the season as long as I stayed away from WOT. Maybe this is when the head of the piston started to go?

  7. #7
    PolarisNut's Avatar
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    Quote Originally Posted by vanlohd View Post
    Polaris Indy Specialty in Shakopee, MN rebuilt the motor. Unfortunatly I've questioned the rebuild since having the motor back. The heads & head gaskets were installed backwards by them and the head bolts were practically loose.
    I've dealt with Dan many times. He's always been a standup guy. He definitely knows how to properly size a bore for an oversized piston...You sure they are excessively loose? Only way to know for sure is to mic them and the cylinder. What did he say when you questioned him about the heads/gaskets? Those pistons are most likely .75mm oversized WSMs. Also, scribe marks on the counterweights wont tell you if the crank is or isn't in phase...it may tell you if the counterweights are properly aligned though. You have to use a dial indicator and a degree wheel for 100% accuracy...there are easier ways that will tell you if you are close.

  8. #8
    Quote Originally Posted by PolarisNut View Post
    I've dealt with Dan many times. He's always been a standup guy. He definitely knows how to properly size a bore for an oversized piston...You sure they are excessively loose? Only way to know for sure is to mic them and the cylinder. What did he say when you questioned him about the heads/gaskets? Those pistons are most likely .75mm oversized WSMs. Also, scribe marks on the counterweights wont tell you if the crank is or isn't in phase...it may tell you if the counterweights are properly aligned though. You have to use a dial indicator and a degree wheel for 100% accuracy...there are easier ways that will tell you if you are close.
    It is promising to hear from someone that has had good experiences with them. I was left feeling uneasy after the heads & gaskets. I never spoke with Dan regarding the heads and gaskets. When I dropped the motor off he was going to contact me and let me know what they were going to do and how much it was going to cost. My next phone call from them was, "Your motor is ready & the total is $xxxx." That didn't sit easy with me either. Maybe it was just an honest mistake.

    Sounds like I should check the phase before taking the cylinders off. Can you point me to a writeup that explains and easier way to check the phase? I'll have to go search the forums and see what I can find. My shop manual only talks about using a dial gauge. Other than the piston almost getting holed and the motor dieing at WOT it seemed to run great. I attribute those problems to a weak fuel pump but I should get my bases covered.

    Oh, I should mention that the last two days I ran the ski I switched from a hotseat exhaust to a factory exhaust. I had to adjust the carbs but I don't remember what I had to do exactly. I think I had to lean it out.

  9. #9
    Moderator beerdart's Avatar
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    A quick and easy method of checking index is bring Mag or PTO to TDC. The other two pistons should measure the same distance. Not very accurate but it should give you an indication if you need to look further.

  10. #10
    Quote Originally Posted by beerdart View Post
    A quick and easy method of checking index is bring Mag or PTO to TDC. The other two pistons should measure the same distance. Not very accurate but it should give you an indication if you need to look further.
    So while the mag or PTO cylinder is at TDC, the other two cylinders should be appx 32.5 mm down from TDC? (65mm stroke divided by 2). Am I understanding this correctly?

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