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  1. #1

    Burned piston. Please help

    I own a '94 Polaris 750 sl. I broke the little nipple that the oil tube goes into at the collar on top of one of the carbs. A local mechanic rebuilt the carbs an stuck the tube up against the fitting and zip tied it on. It looked OK to me. I used the ski once and it ran great. The second time, within about 10 minutes I had hardly any power and it sounded like it was running on two cylinders. I checked my compression and had 100lbs in one cylinder, 105 in another, and 0 in the third. I pulled the head and saw that the piston rings had pretty much burned up. I'm no mechanic, but it was pretty easy to remove the cylinder assembly. What was left of the piston was another story, but I did finally get it off. I assume now that the oil tube was not making a tight enough seal on that collar and that not enough oil (if any) was getting to that cylinder. The connecting rod and crank look OK. I see that I can get a new cylinder assembly and think I can do the job. Should I go ahead and replace the other two, or is that good enough compression? If anyone has any advise please respond. I'm too broke to take to a pro.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Welcome

    Do you know if your compression gauge is accurate? 100 and 105 PSI seem like low numbers, but some inaccurate gauges do read high or low in general.

    Perhaps you could test your gauge on another engine along with a known good gauge, just to verify that your gauge is good.

    Tip: When testing compression, all spark plugs should be out, and throttle held wide open. Battery must also be strong.

  3. #3
    97 Kawasaki STX 1100
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    Quote Originally Posted by danny boy View Post
    I own a '94 Polaris 750 sl. I broke the little nipple that the oil tube goes into at the collar on top of one of the carbs. A local mechanic rebuilt the carbs an stuck the tube up against the fitting and zip tied it on. It looked OK to me. I used the ski once and it ran great. The second time, within about 10 minutes I had hardly any power and it sounded like it was running on two cylinders. I checked my compression and had 100lbs in one cylinder, 105 in another, and 0 in the third. I pulled the head and saw that the piston rings had pretty much burned up. I'm no mechanic, but it was pretty easy to remove the cylinder assembly. What was left of the piston was another story, but I did finally get it off. I assume now that the oil tube was not making a tight enough seal on that collar and that not enough oil (if any) was getting to that cylinder. The connecting rod and crank look OK. I see that I can get a new cylinder assembly and think I can do the job. Should I go ahead and replace the other two, or is that good enough compression? If anyone has any advise please respond. I'm too broke to take to a pro.
    Im guessing you were not holding the throttle open full throttle when doing a compression check if your only reading 100 psi in the other 2 cyclinders... Also was there oil all over the place.. If you had a oil line loose causeing no oil to enter the engine you would have a mess in the hull... If not you have a index problem or a carb problem. As for the cylinder issue thats a easy fix.... But make sure your index and or your carb isnt the cause. Other wise you will be replacing that piston again. I know I learned the hard way!

  4. #4
    bowsniper's Avatar
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    Welcome to the Hulk.

    How long have ya owned the ski? have you done any updates like installing a triple outlet fuel pump? the original pump gets weak over time and causes pistons to go lean and get burned up. usually a hole in 1 of them.

    maybe a fresh hone and rings to fit for the other two.. any pics of the piston tops? are they brown, black, clean or 50/50 ?

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