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  1. #1

    Detonation project-Pair of '95 750 SLT's

    I picked up a pair of '95 750 SLTs pretty cheap from a friend. They're both fried now with detonation and hole in Mag piston. To make it simple I'll only concentrate one at a time. They both had running and stalling issues but this first one would hardly run, so started overhaul first. I found you guys and learned alot real quick..thanks. I upgraded to triple outlet pump, replaced fuel lines, disassembled fuel switch(oring seemed fine, reassembled), rebuilt carbs w/ John Z. kit(he's great too!). I removed and inspected reed cages while I was in there. I was suprised, they looked pristine..every one..I didn't replace carb or reed cage gaskets.. Found this in the water seperator. I thought light liquid would be water, but it's on top?? Syphoned a gallon out the bottom of the tank to see if there was water..seemed OK. Put it back together and went out for 2 hours of fun on the lake...seemed to run great. Then died. When I rebuild carbs, I didn't know how to tune, so decided to note original settings of Low and High and put back to those settings after rebuild. Thought I'd see how it ran and adjust from there.
    Here's the settings I used: Low M=1-1/8, C=7/8, P 1; High M=1, C=1/2, P=3/4
    Another thing, I checked the water seperator after it died and found same mystery liquid floating on top of the gas?? I use 91 octane gas from street pump. I did add a can of SeaFoam to full gas tank...could that be in the seperator?
    Here's my questions:
    1. Where would you recommend I get a piston?
    2. How do I know what size?
    3. Do I get cylinder off and checked out first? Maybe needs recondition and then oversized piston?
    4. Suppose you're going to suggest I pull engine and check Index and Flywheel Key. Do I have too?...I'd rather not pull engine.
    5. Are these Fuji engins made of Crack? I think I'm addicted...let's get busy!
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  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Um, are you running it with a damaged piston?

    Did you drain out the old fuel and clean the tank? If not, no idea what might be in there.

    My opinion; if someone thinks something needs Seafoam, it actually needs maintenance, cleaning or repair. Old fuel in the tank is just a gamble.

  3. #3
    BlueFishCrisis's Avatar
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    I would drain the tank as well. No telling what those two components are.......

    I would assume these pics are from before your ride, as you mentioned the piston is now holed.

    There are several online sites for picking up a replacement piston. Simply google search what you are looking for. WSM makes a good replacement. If you don't know what size pistons you have, you will need to measure them with a set of calipers. Stock bore is 69.75mm. If the walls are in good shape, you should be able to simply hone and install new pistons and rings.

    Stock settings are lows 1/2. Highs M 1-1/4 C 3/8 P 7/8

  4. #4
    I didn't tell you that I had a ski "mechanic" tell me to add seafoam when I first got the skis and they were running rough. They were running rough-no low end and then just die at high speed. He supposedly replaced stators in each for $275 each. Then he sold me a stolen trailer. I don't think he repaired anything...I got ripped off and an education. I called to local reputable dealers...they said nobody works on Polaris because they're out of production. That's when I decided I'll need to do it myself. I've run through all but a few gallons of the original gas(with added seafoam) and here's where I'm at. After repairs, it ran great, 45mph no stalling...thought I had it fixed. After 2 hours of fun on the lake it died..I checked plugs and saw powdery gray...told me there was a big problem, so it took off the head. engine turns freely by hand and seems no crank play. I plan to drain and clean tank. I plan to replace piston (how do I know size? and where's a good source?) Any other info or picsThanks for your help.

    (ps: same on second ski??...except I've run about 2 tanks through it because it's been more reliable. It had no low end but but got to top speed pretty reliable...then just die in the water. It would generally start right back up. At that point, I hadn't even upgraded fuel pump or rebuilt cards. on it...seemed it was running OK enough to have fun while I was figuring out they other one. Funny, the day they both died was the first day I ran it with the upgraded pump?? It also ran great and didn't die at top speed until later in the day. When I found big problem with first one I checked the one and same thing?

  5. #5
    BlueFishCrisis's Avatar
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    Welcome to the Hulk! You should set your carbs back to factory after making the repairs needed as a good base line. You can get a great education by reading everything here: http://polarispwcknowledge.shorturl.com/

  6. #6
    These pics are after "ride of death" Might be hard to see but deep divot in Mag..the detonation damage killed it. (the other ski actually has hole and detonation damage). I'll have cylinder checked out and honed, replace cylinder, drain fuel tank and reset carbs.
    I understand lean mix, but don't understand detonation and how to prevent. I read post and still confused about that. Any thoughts on reason I have this problem?

  7. #7
    BlueFishCrisis's Avatar
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    I can see in the first picture what appears to be a divot, and it looks like the piston has eroded at the exhaust side. The piston dome is all black, unlike the other two which are showing a few areas of wash at the ports. This appears to be a lean condition burn. This can be caused by inadequate fuel deliver due to bad fuel pump, blocked lines, bad carb diaphragm, stuck needle/seat in carbs, main crank seal leak, other air leaks after the carbs in the intake tract, or out of index crank. You will need to rule these out one at a time, and it sounds like you have started with replacing the fuel pump and rebuilding the carbs......

  8. #8
    Sooo...how much for the 780? stimpsonjcat's Avatar
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    I agree, lean condition. One of my active skis has a similar divot but seems to have stabilized to a better wash pattern now. It will be getting torn apart and repaired soon.

  9. #9
    Can you take a look at the cylinder just above the exhaust port...there's scarring from where the ring broke. You can barely feel it with your finger. It's hardly raised at all. Is this something that can be easily smoothed out and honed? Or am I looking at having machined? Or would used or new cylinder be a better option? I'd like to stay with stock piston rather than bore and oversize piston...or is that not a big deal?
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  10. #10
    BlueFishCrisis's Avatar
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    Alot of people have run one piston oversized vs the rest, especially for simple rec riding. You can try a hone, but all a hone really does is deglaze the cylinder walls. There really isn't any way to smooth out something that has removed material. You may be able to run a new piston and rings in there, but will definitely lose compression points with damage to the cylinder. The only question is how much.....

    Have you looked into an SBT swap?

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