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  1. #1

    Tricky Timing Question

    Currently the timing mark is dead center on crank seam.PTO is exactly 2 degrees short but within tolerance (just barley)

    Hi Everyone,... been working on this SLT 780 with exploded top ring landing damage on the PTO. (I discovered that whoever had this machine before me split the head gasket (maybe it was compromised by lack of proper clearance with the head)

    Anyway, I checked the crank index 5 ways and discovered it to be off by two degrees short (retarded) ATDC from the front two cylinders every time.

    My question:

    Is two (2) degrees out of index a problem? Does it need to be rebuilt?

    Although the shop manual says +/- 2 degrees is within tolerance, I wonder if I should compensate and adjust the timing for this "limiting" crank index factor. And, if so in which direction should I adjust the stator and by how much?

    Any suggestions would be greatly appreciated.

    Bobby

    p.s . I used this proceedure (below) to determine crank index:

    Disconnectbattery ground cable and all spark plug high
    tensionleads; ground high tension leads to engine. Disconnect lanyard from engine stopswitch before proceeding with the following steps.
    1 .Securely fasten a degree wheel on the flywheel or
    outputcoupler/driveshaft. Use a large degree wheel
    for moreaccuracy, and make sure it is mounted
    concentricallywith the crankshaft center line. Cut a
    slot in apaper or cardboard degree wheel and slide it
    over thedriveshaft if engine and pump shaft is
    installedin craft. Again, be sure to secure the degree
    wheel.
    2 . Sharpena coat hanger or section of welding rod and
    anchor itto a convenient spot. Point the sharpened
    end at theouter perimeter of the flywheel or drive
    couplermounted degree wheel.
    3 . Installa dial indicator into the magneto end cylinder
    spark plughole (front) (#1). (The ignition timing is
    referencedby the magneto end.)
    4. Rotatethe engine to bring the piston to top dead
    center(TDC) on the cylinder with the indicator
    installed.
    5 . LocateTDC as accurately as possible by finding the
    center ofthe point where there is no piston
    movement."Zero" the dial indicator at this point.
    Continue torotate the crankshaft in the normal
    directionof rotation (counter clockwise as viewed
    from therear of the craft) until the dial indicator reads
    .100"(2.54mm) after top dead center (ATDC).
    IMPORTANT Do not allow the crankshaft to move
    from thisposition.
    6 . Bendthe pointer or move the degree wheel until the
    pointeraligns with the 180 or 120° mark on the
    degreewheel.
    7 . Withthe pointer aligned, make sure the degree wheel
    and pointerare secured and will not move out of
    position.Re-check accuracy of this location by
    repeatingsteps 4 and 5. The pointer should align
    with the180 or 120° mark when the dial indicator
    reads.100" (2.54mm) ATDC.
    IMPORTANT Do not move the degree wheel or pointer
    after theinitial setting on the mag end cylinder - simply
    read thewheel and dial indicator.
    8 . Removethe dial indicator and install in cylinder #2
    (center).Repeat steps 4 and 5 for cylinder # 2. Note
    the degreewheel indication when the dial indicator
    reads.100" ATDC. It should be 180 or 120° (± 2°)
    fromcylinder #1. Proceed to PTO cylinder (#3).
    Cylinder #3should also be 120°<± 2°) from cylinder
    #1.
    Symptoms ofan out of index crankshaft can include:
    •Difficulty calibrating carburetor (repetitive plug
    fouling onone cylinder with no other cause);
    •Unexplained piston failure on one cylinder (i.e.
    severedetonation, broken ring lands, piston holing);
    • Excessivevibration of engine, backfiring, etc.;
    • Rough idle, poor top speed.
    Last edited by polaris780slt; 09-28-2012 at 02:19 PM.


  2. #2
    I'm addicted to Polaris PWC ghostinstallations's Avatar
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    I would leave it as is. 2 degrees is within spec and you should notice no ill effects as a result.

    Maybe you misspoke, but the reading should NOT be from TDC, but past TDC.

  3. #3
    Quote Originally Posted by ghostinstallations View Post
    I would leave it as is. 2 degrees is within spec and you should notice no ill effects as a result.

    Maybe you misspoke, but the reading should NOT be from TDC, but past TDC.
    Ooops.. sorry, you are correct. ATDC is what i should have said...I'll fix the post and thanks for the advice

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