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  1. #1
    97 Kawasaki STX 1100
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    HELP- pop off pressure 95 polaris 750 sl

    Hey guys, I have major problems. Ive melted my 3rd now PTO piston in the past 3 weeks. Ive removed the engine replaced all the seals and used three bond to reseal the case and still melted another piston. I removed the carbs this evening and did a pop off pressure check and PTO is 18psi center is 16psi and front is 17psi... Specs say 10-18psi but Ive herd 18 is way to high any thoughts??? What should it be... Is this the cause or any other suggestions.... 95 polaris 750 SL hole in the pto piston for the 3rd time in 3 weeks. Im getting fustrated.... I can feel the engine at regular cruise start to bog down and when I get in the throttle hard it comes back to life. At least it did until the hole..


  2. #2
    ryandi2's Avatar
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    Quote Originally Posted by mogyver69 View Post
    Hey guys, I have major problems. Ive melted my 3rd now PTO piston in the past 3 weeks. Ive removed the engine replaced all the seals and used three bond to reseal the case and still melted another piston. I removed the carbs this evening and did a pop off pressure check and PTO is 18psi center is 16psi and front is 17psi... Specs say 10-18psi but Ive herd 18 is way to high any thoughts??? What should it be... Is this the cause or any other suggestions.... 95 polaris 750 SL hole in the pto piston for the 3rd time in 3 weeks. Im getting fustrated.... I can feel the engine at regular cruise start to bog down and when I get in the throttle hard it comes back to life. At least it did until the hole..
    have you repalced the fuel pump? cleaned the carbs? replaced the fuel lines? did you inspect your fuel shut off valve?

    did you do a leak down test on your motor?

  3. #3
    If you have a hole in the center of the pistion... that's a good indication of pre-ignition.

    Did you shave the heads for more compression? Is your crank indexed correctly? Is your timing set correctly?


    Since it's your PTO... I'd say you have a twisted crank and/or your ignition timing is too high for the fuel you are using. Also... if it was a lack of fuel... chances are you would "stick" a piston, and not just burn it.

    Speaking of that... are you at least using a "mid-grade" fuel? 87 oct doesn't work anymore since they are putting in 10% alcohol.

  4. #4
    OK... I re-read your post. Did you burn a hole in the top... or did you seize it?

    If you burnt it... see my above post. If you melted the sides, then it was fuel related.


    OK... you say that you re sealed the case... but did you do a leak-down on the engine? Just because it has new seals, doesn't mean one of them got nicked, and is leaking. OR... that the crank bearing on the PTO side is in perfect condition. If it's wobbling a little, the seal could fail quickly. (causing a lean run)

    18 psi isn't too high, as long as it's tuned for that pressure. I run 43 psi in my 720 seadoo engines. (38 SBN carbs)

  5. #5
    97 Kawasaki STX 1100
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    Quote Originally Posted by Dr Honda View Post
    OK... I re-read your post. Did you burn a hole in the top... or did you seize it?

    If you burnt it... see my above post. If you melted the sides, then it was fuel related.


    OK... you say that you re sealed the case... but did you do a leak-down on the engine? Just because it has new seals, doesn't mean one of them got nicked, and is leaking. OR... that the crank bearing on the PTO side is in perfect condition. If it's wobbling a little, the seal could fail quickly. (causing a lean run)

    18 psi isn't too high, as long as it's tuned for that pressure. I run 43 psi in my 720 seadoo engines. (38 SBN carbs)
    Let me back up alittle:
    First I bought the ski, replaced the spark plugs and put a new battery in. 2 days later I melted the PTO piston. The hole side of the piston was melted at the exhaust side of the piston.
    Then I noticed the rear pto seal was sitting cock eyed so I removed just the outer seal and replaced it. Put another piston in and 2 days later again melted another piston.
    This time I removed the engine completely. Split the cases, replaced all the 3 seals (front and the 2 rear seals) and resealed the case with threebond. When I was reinstalling the stator I made sure the timing mark notches lined up with the case. I then checked the adjustments on the carb front 1 turn center 1/2 and rear 3/4 for the 95 750 sl. I never did do a pressure check on the engine once it was sealed. I was pretty confident in my seal job. I guess I should of just to say I did though...

    I have noticed now though when you do burn a hole and loose compression because the air is able to travel from one side of the piston to the other that rear seal has moved again from the pressure build up in the case im guessing.... I keep thinking I have a plugged passaage in the carb or something. This carb has had alot of work. The choke has been removed and the inside has been ported and polished. A prime kit was added and the oil injection was added to the other side of the carb in the choke hole location... Im just not sure if any jetting was done , or if the fuel filter is dirty or what...

  6. #6
    97 Kawasaki STX 1100
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    Quote Originally Posted by ryandi2 View Post
    have you repalced the fuel pump? cleaned the carbs? replaced the fuel lines? did you inspect your fuel shut off valve?

    did you do a leak down test on your motor?
    Yes, New mikuni 3 outlet fuel pump, new fuel lines, return restictor is in place. All new seals in the bottom end. 18 psi pop off pressure. No I havent rebuilt the carbs... 130 psi compression before melting the piston.

  7. #7
    the plot thickens......

    Did you verify the timing, or did you just set it to the mark? (since you melted the side of the piston, it's a moot point)


    OK.. the crank seal is pushed out again. That's bad. Even a little... it can now be drawing air, and going lean. This is why a leak down is so important. With 10 psi in the engine... you may have found that problem before you ran it.


    If the carbs have been modified at all... they need to be re-jetted. The factory specs won't work anymore. The combustion temps have gotten WAY too high. I would start by bringing the main jet in that carb up 2 sizes. If it's a Mikuni, they run 2.5 per size. And for good measure... I'd bring up the mag, and center at least one size.

  8. #8
    97 Kawasaki STX 1100
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    Quote Originally Posted by Dr Honda View Post
    the plot thickens......

    Did you verify the timing, or did you just set it to the mark? (since you melted the side of the piston, it's a moot point)


    OK.. the crank seal is pushed out again. That's bad. Even a little... it can now be drawing air, and going lean. This is why a leak down is so important. With 10 psi in the engine... you may have found that problem before you ran it.


    If the carbs have been modified at all... they need to be re-jetted. The factory specs won't work anymore. The combustion temps have gotten WAY too high. I would start by bringing the main jet in that carb up 2 sizes. If it's a Mikuni, they run 2.5 per size. And for good measure... I'd bring up the mag, and center at least one size.
    Ok, The seal issue only pops out after the engine burns a hole and I try driving it back to shore on 2 cyclinders.... The carb issue is a hole nother story. First off I pulled the carbs apart today did a pop off check and disassembles the carbs. I found the needle and seat on the PTO carb dirty dirty dirty. The other 2 carbs were great.... I cleaned all 3 carbs. blew all the fuel passages with a air hose and relubed with WD before reassembling.... Re checked pop off everything is good. Im hoping That dirty crap in the needle and seat may of been my troubles.... But honestly I think Im just gonna buy a used stock carb on ebay and rebuild it rather then using this carb since I know nothing about whats been done to it other then whats obviuos with the eye..

  9. #9
    97 Kawasaki STX 1100
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    Quote Originally Posted by Dr Honda View Post
    If you have a hole in the center of the pistion... that's a good indication of pre-ignition.

    Did you shave the heads for more compression? Is your crank indexed correctly? Is your timing set correctly?


    Since it's your PTO... I'd say you have a twisted crank and/or your ignition timing is too high for the fuel you are using. Also... if it was a lack of fuel... chances are you would "stick" a piston, and not just burn it.

    Speaking of that... are you at least using a "mid-grade" fuel? 87 oct doesn't work anymore since they are putting in 10% alcohol.
    The motor seams to be completely stock only 130 psi all 3 cyclinders. The only thing that appears obvious is someone messed with the carbs and removed the choke,. added primer and oil injection to the carbs and replaced the spark arresstors with after market... And ported the inside of the carbs... I cant do much of a crank index test since I already have the piston out but I did check the font and center indexing and those 2 cyclinders seam to be ok... I will check all 3 together once I get my new piston. I did connect a screw driver to the rear of my crank and try to check for true ness. It appears to be straight but I know I should use a dial indicater will try to see if I still have one hidden in my tools I can hook to this application maybe tomorrow...

  10. #10
    bowsniper's Avatar
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    ill sell ya a 750 rack for 75.00 plus shipping. stock stuff. with oil pump or without. comes with studs, gaskets and nuts,3 carbs,rack,choke plates removed already.plug or primer plugs are there,clean... stock main 120 jets and 90's pilots. om me if interested.

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