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  1. #1
    bnasty's Avatar
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    1995 SLT 750 - Motor failed AGAIN!!

    Hey guys,
    Last season, my PTO piston blew, and I rebuilt the top end, installing a triple outlet mikuni pump, and converted over to pre-mix. After 2-3 hours of break-in, the same piston burned through again. I have yet to tear it back down, but I will in just a couple weeks.
    I need to know if anyone else has been through the same issue, and what they did to correct the problem. Here is what was done during the rebuild:

    Triple Mikuni fuel pump conversion
    Honed all three cylinder sleeves, and measured with a micrometer
    installed all new fuel line (the clear blue stuff)
    installed new pistons, wrist pins, bearings, lubed thoroughly
    rebuilt all three carbs
    made sure the cooling system had no restrictions

    Break in procedure used 20:1 premix, ran 10 min, cool, 15, cool, 20, cool, etc, up to a full hour, left it sitting for a week. Took it back out, ran for 20, cool, 45, cool, 1hr, cool, and took it up to full throttle, and about 20 seconds into WOT, all power was lost!

    Let me know what yall think, and some of you guys that helped last year may still be around!
    -Brendon


  2. #2
    la90043's Avatar
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    dude.gee. what a bummer. pull the domes off and lets see how it reads with some pics..................

    Quote Originally Posted by bnasty View Post
    Hey guys,
    Last season, my PTO piston blew, and I rebuilt the top end, installing a triple outlet mikuni pump, and converted over to pre-mix. After 2-3 hours of break-in, the same piston burned through again. I have yet to tear it back down, but I will in just a couple weeks.
    I need to know if anyone else has been through the same issue, and what they did to correct the problem. Here is what was done during the rebuild:

    Triple Mikuni fuel pump conversion
    Honed all three cylinder sleeves, and measured with a micrometer
    installed all new fuel line (the clear blue stuff)
    installed new pistons, wrist pins, bearings, lubed thoroughly
    rebuilt all three carbs
    made sure the cooling system had no restrictions

    Break in procedure used 20:1 premix, ran 10 min, cool, 15, cool, 20, cool, etc, up to a full hour, left it sitting for a week. Took it back out, ran for 20, cool, 45, cool, 1hr, cool, and took it up to full throttle, and about 20 seconds into WOT, all power was lost!

    Let me know what yall think, and some of you guys that helped last year may still be around!
    -Brendon

  3. #3
    Maulin Marto's Avatar
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    piston blew...

    Quote Originally Posted by bnasty View Post
    Hey guys,
    Last season, my PTO piston blew, and I rebuilt the top end, installing a triple outlet mikuni pump, and converted over to pre-mix. After 2-3 hours of break-in, the same piston burned through again. I have yet to tear it back down, but I will in just a couple weeks.
    I need to know if anyone else has been through the same issue, and what they did to correct the problem. Here is what was done during the rebuild:

    Triple Mikuni fuel pump conversion
    Honed all three cylinder sleeves, and measured with a micrometer
    installed all new fuel line (the clear blue stuff)
    installed new pistons, wrist pins, bearings, lubed thoroughly
    rebuilt all three carbs
    made sure the cooling system had no restrictions

    Break in procedure used 20:1 premix, ran 10 min, cool, 15, cool, 20, cool, etc, up to a full hour, left it sitting for a week. Took it back out, ran for 20, cool, 45, cool, 1hr, cool, and took it up to full throttle, and about 20 seconds into WOT, all power was lost!

    Let me know what yall think, and some of you guys that helped last year may still be around!
    -Brendon
    rear crank seal? Did you spray any carb and choke near it before you ran it in the water? small crack in the exhaust man. getting water in the rear hole under full load? I would bet the rear seal area leaking / sucking air...my 2 cents. How did the piston look? Why the need for such a heavy mix on the oil-fuel? 20-1 with the injector too? Just wondering? the more oil used in the fuel, the more you displace the fuel molecule, thus making it lean?.....old school lesson I learned a long time ago.

  4. #4
    la90043's Avatar
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    should it not be 40:1

    Quote Originally Posted by Maulin Marto View Post
    rear crank seal? Did you spray any carb and choke near it before you ran it in the water? small crack in the exhaust man. getting water in the rear hole under full load? I would bet the rear seal area leaking / sucking air...my 2 cents. How did the piston look? Why the need for such a heavy mix on the oil-fuel? 20-1 with the injector too? Just wondering? the more oil used in the fuel, the more you displace the fuel molecule, thus making it lean?.....old school lesson I learned a long time ago.

  5. #5
    la90043's Avatar
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    when i broke in my 750 i added one quart of oil to a full tank of fuel with the oil pump running

    Quote Originally Posted by la90043 View Post
    should it not be 40:1

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by bnasty View Post
    ...converted over to pre-mix...20:1 during break-in
    Quote Originally Posted by Maulin Marto View Post
    ...Why the need for such a heavy mix on the oil-fuel?
    ...the more oil used in the fuel, the more you displace the fuel molecule, thus making it lean?.....old school lesson I learned a long time ago.
    Quote Originally Posted by la90043 View Post
    should it not be 40:1
    Pre-mix break-in oil ratio is normally 32:1, then pre-mix 40:1 after break-in.

    Were you checking piston wash or spark plug condition during break-in?

    Were the needles and seats replaced when the carbs were rebuilt?
    What carb settings were you using?

    What do the other two pistons look like?

    Pictures when you pull the heads?

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    What do you mean piston blew? Does it have a hole in it?

    If there's a hole, you ran lean (lack of fuel).

    Did you ever run a leak down test. (air leak)

    Have you done a crank index? (piston timing issue)

    What are the carb screws set at right now?

    Did you check the burn characteristics while performing the break in? (read plugs or piston wash)

  8. #8
    bnasty's Avatar
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    Sorry, I meant 32:1, not 20:1!! And that was without injection, as I removed the oil pump and installed blockoff plate.

  9. #9
    bnasty's Avatar
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    Quote Originally Posted by xlint89 View Post
    What do you mean piston blew? Does it have a hole in it?

    If there's a hole, you ran lean (lack of fuel).

    Did you ever run a leak down test. (air leak)

    Have you done a crank index? (piston timing issue)

    What are the carb screws set at right now?

    Did you check the burn characteristics while performing the break in? (read plugs or piston wash)
    Yes, blew=hole in piston. I am assuming that this was the issue, need to perform leak down test, how to??

    Carb screws of the front two cyls were set just past factory, maybe 1/10 of a turn past, because I wanted it to run slightly rich. PTO was set at a half a turn past factory.

    Piston wash and plugs looked fine all the way through the breakin, and just before another hole was burnt through the piston.

    Do you think the leak down test should be the first thing checked? I would have to put a good piston in the cylinder before doing the test, correct?

  10. #10
    bnasty's Avatar
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    Quote Originally Posted by K447 View Post
    Pre-mix break-in oil ratio is normally 32:1, then pre-mix 40:1 after break-in.

    Were you checking piston wash or spark plug condition during break-in?

    Were the needles and seats replaced when the carbs were rebuilt?
    What carb settings were you using?

    What do the other two pistons look like?

    Pictures when you pull the heads?
    I should get some time to pull the heads this coming week, this weekend is far too busy!! Ill post them asap.

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