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  1. #1

    Last Effort (2) SLX780's - Need Ideas


    Its been a while but hopefully some of you will remember my situation. I have two SLX780's which both sat for over 5 years. Both took out a piston within the first 10 minutes of the first ride outta storage. With ya'lls help I was able to rebuild both top ends.

    Things I have done:
    1.) checked index - Good
    2.) tore down carbs and cleaned - Spotless
    3.) pressure tested carbs(leak down) - good
    4.) Cleaned out gas tanks- spotless
    5.) Cleaned pt cock - spotless and functional
    6.) checked but didnt replace gas lines (still like new) -
    7.) Did the raquetball leak down test (sucessfully I hope) - had trouble with getting a good seal but i think I am ok there -
    8.) tore the carbs down again and replaced the orings -

    But - when i put them in the water I managed to pop the pistons again in short order. After an hours riding time. Lean cylinders.

    SO . . .
    9.) the last thing I did before giving up last fall was tear down the carbs and clean again. Also changed out all the orings. -
    10.) Rebuilt the fuel pumps with rebuild kits even though I dont think there was a problem with the pumps -
    11.) Reeds look like new -
    12.) Set all carbs to run rich side of setting specs - havent run yet since the adjustments -

    I havent yet run the machines since the last carb clean and fuel pump rebuild. One machine has a cylinder that will soon need replacing from the earlier lean condition. The other has yet to be run since its second rebuild. Ideally, I would love to figure out for sure whats making the cylinders go lean on the fresh clean good cyl machine before I smoke it again. What ever is wrong with one is likely the problem with the other as well.

    Long story - Sorry guys!! So, obvious question "What the hell am I missing?" Sure would appreciate someone in the know giving me some ideas cause Im really at a loss. Truth is I might have fixed the problem by setting all the carbs at the fat side of the settings but Im scared now to run them for fear of cratering more cylinders. This stuff is too expensive and time consuming to keep destroying.

    Thanks in advance.
    Last edited by Lockout; 04-27-2009 at 11:50 AM. Reason: none

  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    Sounds like you've done quite a bit already.

    Why haven't you checked piston wash when breaking the ski in? Kinda tells you how things look internally.

    Which piston is failing? When you say piston failure from running lean, you mean a hole in the center correct?

    You've checked crank index, but have you ever checked the timing? Possibly sheared the woodruff key and advanced the timing

    Did you verify jet sizes, and which carb they're in? Sometimes the MAG and PTO need more fuel than the CEN.

    How about a pic when the heads are off. Often guys can spot something amiss in a pic.

  3. #3
    Kevin right? I remember you from last year. Thanks for the reply. Actually I checked index using a method you taught me on both skiis.. It checked out ok. The Pistons were badly scored and it appears like the outer edges closest to the walls breaks away. Not really a hole in the center ever. I was checking wash but like I mentioned they fry pretty quick. On one machine it was the pto. on the other it was the mag. These machines i bought new in NY. They are all stock. The last time I worked on them I rebuilt the fuel pumps and I believe it was you that gave me the carb settings to set. If i remember the center was set leaner than the pto or mag. AS far as timing...i was under the opinion that the index check indicated proper timing? The machines ran great for a half hour or so after i rebuilt them. It was hard to see piston wash looking into the plug holes. Anyway, after a tank of gas i pulled the heads and found i had burnt them up again.

  4. #4
    The pics are in my posts from last year. I gotta go check and see if they are still on here by searching my screen name. Its been a while. Ive been overseas.

  5. #5
    Lake Mead Bum & BTLS Mark starflight's Avatar
    Join Date
    May 2007
    Boulder City, NV
    Quote Originally Posted by Lockout View Post
    It was hard to see piston wash looking into the plug holes.
    If you have the piston at bottom dead center, and have a flex light you can check wash w/o pulling the heads. The light in the pic below is about $8 at the hardware store, and it will fit through the plug hole so you can see whats going on.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    Yes, it's Kevin.

    The light Starflight has a pic of, I actually use myself. Home Depot carries them for $8 like he said.

    Jetting suggestions you get from anyone on here are just that, suggestions. It's imperative that you check your piston wash before doing any extended WOT riding.

    Tuning is the part that sucks. Assembly is only 30%, tuning is the other 70%.

    Crank indexing is different from engine timing. Crank index, just verifies the pistons reach TDC at exactly 120* apart from one another. (granted, a piston out of phase will change THAT cylinders timing, because it is now either more ahead or behind the other 2)

    Engine timing needs to be checked with a timng light. It can be altered if the woodruff key shears, and the flywheel spins ever so slightly on the crankshaft.

    Has your elec. stator ever been removed? If someone removed it at some time, it's possible it wasn't put back on the right mark. That will affect engine timing and cause serious issues.

    Do you cruise at 3000 RPM alot? That's when the CDI goes into full advance (timing) and if you don't have enough fuel, can cause the same issue.

    Are these the pics?

  7. #7
    BBCaprice's Avatar
    Join Date
    Apr 2007
    Titusville FL
    The Pistons were badly scored and it appears like the outer edges closest to the walls breaks away. Not really a hole in the center ever.

    Could be detonation if the outer edges of the pistons are broke. Was there any aluminum "slag" on the heads?

  8. #8
    Rasta Mon Condoms We Be Jammin!!!!! TxPro1200's Avatar
    Join Date
    Aug 2006


    sounds like its hot or lack of oil,is just the mag or all 3 cylinders??

  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Can you post up clear photos of each piston top and cylinder wall?

    Hint: Use close-up mode on your camera, often marked with a flower icon.

    When you upload photos, high resolution is OK. Upload a couple of pictures at a time, until your post is complete, then Preview and Submit.

  10. #10
    Yes, those were the pics of the first machine i tore into. The second machine looked basically the same. Both machines top ends have been rebuilt. One has yet to be run. The other has been run and it appears to have started to burn up the mag cylinder already. Im obviously going to replace that one again but not until i figure out why they are cooking. So concentrating on the machine that was just rebuilt. All three pistons are new and installed. Machine is assembled and basically ready to ride. I can start it up and go run it but I know its prolly gonna burn up. I would really like to avoid that if at all possible. Should I just run it slow for 30 minutes and check the wash? If I do that will it overheat and burn? I dont have a timing light or know how to do it, but I can get one.

    Kevin - the answer is no has never been removed.

    Could there be some tank vent or something that causes them to go lean?

    When the machines run correctly I generally run at full throttle (who doesnt right? ) So, 3000 or better I would guess.

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