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  1. #1

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    99 Polaris SLTH 700 Piston Burn issue..HELP

    I recently purchased this 99 SLTH 700 with a hole in the Mag side piston. I bored .5mm over and replaced both pistons. In trying not to repeat the hole in piston, I replaced the fuel lines, Cleaned the carbs, and sprayed carb cleaner around the rear crank seal when I initially started it to check for a leak in the seal. Took it to the lake and ran it for 5 minutes at a time removing the plugs and checking the pistons with a flashlight and they looked good every time. After doing this about 5 times i tried to drive it a little longer. It kept bogging when trying to accelerate but would finally take off yet seemed to have a miss. After about 10 minutes it shut off and wouldn't restart. I checked compression first thing and got only 30 lbs in the mag piston and 140 in the rear.
    Any ideas on where i went wrong would much appreciated. this is getting expensive quick.
    thanks much,
    troy


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

    Were you doing the break-in process on your new top end?

    What were you looking for when you were checking the piston tops?

  3. #3

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    I had added oil to the fuel to help with the break in and kept it under 5000 rpm. I was checking the piston top to make sure I wasn't burning another hole.....not knowing exactly what had caused this with the previous owner.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    General Break-in procedure

    Have you confirmed that the new piston now has a hole?

    Piston holes are caused by lean fuel delivery at medium and high throttle levels, and mostly at near WOT.

    If you were staying below 5,000RPM, and you still holed the piston, then the fuel delivery was very lean.

    Were the carb jets and screw settings checked, and matched the factory specs?

  5. #5

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    It only actually burnt the edges of the piston this time. Itwas on both sides of the piston though. I have a pic but can't seem to figure out how to attach one to this thread. I didnt touch any of the carb settings. The only two screws (I beleive are low screws) have a little tab 0n them to keep from any major adjustment.....maybe a quarter turn at most. regardles i didn't move them at all.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by Theweavercrew View Post
    It only actually burnt the edges of the piston this time. It was on both sides of the piston though.

    I have a pic but can't seem to figure out how to attach one to this thread.

    I didn't touch any of the carb settings. The only two screws (I believe are low screws) have a little tab 0n them to keep from any major adjustment.....maybe a quarter turn at most. regardless i didn't move them at all.
    Detonation, or foreign object damage?
    Is the cylinder head damaged?

    What octane gasoline were you using?

    To attach photos, click reply, then chose Advanced Mode. Scroll down to Manage Attachments. Select and Upload your JPG images.
    Click Preview to confirm there are there,.

  7. #7

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    Regular 87 octane. The head looks good.
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  8. #8
    casey67's Avatar
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    Spraying carb cleaner at the rear of the engine (pto0 will not help diag the front (mag) cylinder. The front seal is in the cover,you would have to take out the timing plug,than spray some carb cleaner.

    From the pictures,it looks like you are lean. You need to look for leaks,and also look at the fuel pump(mounted to the rear carb,and supplies fuel to the rear carb first) if it is weak/leaKING you will never have enough fuel to the front cylinder.

  9. #9

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    That makes good sense on the pump. Would you know if the carb rebuild kit comes with the fuel pump diaphram or is it a seperate item. Also, is their a pump in both carb's?
    I appreciate the tip on removing the timing plug. I had just figured that their was no easy way to check the seal on that end.

    I really appreciate the help.
    troy

  10. #10
    Click avatar for tech links/info, donation request K447's Avatar
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    Hard to tell from the slightly fuzzy photos, but are you sure there is zero damage to the head?

    Those pieces of piston had to go somewhere when they broke off.

    The damage doesn't look so much like detonation as it does mechanical - huge chunks of piston edge are missing.

    What type of rings does that piston use? Is one ring tapered? Could the upper ring have been fitted incorrectly?

    Another thought is that there is some vertical play in the connecting rod ends, and the piston was actually contacting/hitting the cylinder head around the perimeter of the piston.

    For clear close up pictures, use close-up mode on the camera, often depicted with a flower icon.

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