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Uh oh, jumped cam chain???

Discussion in 'The Garage' started by wannabridin, Mar 24, 2020.

  1. wannabridin

    wannabridin XS650 Guru

    Well I'm infinite Idiocracy, I might have jumped teeth on the cam chain...

    While changing the cam chain tensioner, I absent mindedly spun the flywheel. Wondering what the clicking sound was, and why the flywheel was turning so easily (both directions mind you...) I had the sudden heart sunken feeling... "I just jumped XX number of teeth like an IDIOT!"

    Am I TOTALLY boned (top end job?), Or can I "jump" the chain/sprocket back to stock position the same was I removed it??

    Attached Files:

  2. kevski

    kevski XS650 Addict

    kent uk
    I am not sure and this is not gospel that can happen on these as the chain is a really tight fit, put tensioner back in and adjust as the manual says, remove valve covers, spark plugs, and turn over very slowly to top dead centre there is a pin on the right hand end of the cam that locates the advancer unit this should be either pointing straight up or straight down according to which piston is on its firing stroke, i am sure someone else will correct me if this is not right.
    Last edited: Mar 25, 2020
  3. Downeaster

    Downeaster Everything in XS Top Contributor

    First thing to do is verify that you did indeed jump the sprocket.

    Ignition off, spark plugs out, take off the cover on the left side case and all four valve adjustment covers and SLOWLY turn the engine over until the timing marks line up.

    Check to see that all four rocker arms are a little loose indicating that all four valves are closed. Peek in the spark plug holes and see that both pistons are at the top.

    If so, all good. If not, you'll have to reset the cam timing. Easiest in the long run to do that with the engine out of the frame.

    Pop the cam cover off

    Find and remove the cam chain master link (may have to grind off the rivets)

    Wire the chain so it doesn't fall into the crank cases,

    Free the chain from the cam gear and let it droop so it's free of the crankshaft gear.

    Roll the engine over to TDC

    Line up the timing marks on the cam

    Reattach the chain with a new master link.

    MIght as well check the front cam chain guide and the shoe on the chain adjuster while you're in there.

    Not technically difficult, just fiddly and time consuming.
    timbeck and gggGary like this.
  4. kevski

    kevski XS650 Addict

    kent uk
    Without any disrespect to downeaster, if motor is not in the frame there will be no need to split the chain, as without the tensioner in the cam bearings can be carefully slid off allowing the chain to be moved to the correct position, how ever you decide to do, just double check everything and use a manual.
    Downeaster, timbeck and gggGary like this.
  5. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    Scary cuz (I think) it's pretty easy to bend a valve with a piston. If the cam is at TDC you can safely rotate the crank and you will not hit a valve.
    Hint; this may allow you to get the cam, crank back to the correct relationship without an engine-ectomy, because gravity sez the chain is slack. loose, moveable, at the crank sprocket not the cam sprocket.
    Downeaster and timbeck like this.
  6. Downeaster

    Downeaster Everything in XS Top Contributor

    True. I'd forgotten about that. Thanks for the reminder.
    gggGary likes this.
  7. Downeaster

    Downeaster Everything in XS Top Contributor

    Also true and worth a try. I tend to just go for the worst case scenario sometimes.
    gggGary and MaxPete like this.
  8. MaxPete

    MaxPete Lucille, Betty & Demi - I suggest but THEY decide. XS650.com Supporter Top Contributor

    ....the gloomy outlook is because of the weather where you live DE.
    gggGary and Downeaster like this.
  9. Downeaster

    Downeaster Everything in XS Top Contributor

    That, and extensive experience with that potlicker Murphy...
    gggGary, MaxPete and GLJ like this.
  10. grizld1

    grizld1 Grumpy old man Top Contributor

    There is no point at which all four valves are closed unless the camshaft has been modified to fire both cylinders simultaneously. In a 360* twin, when Cylinder One is at TDC on the compression/power cycle (both valves closed), Cylinder 2 is at TDC on the exhaust/intake cycle (both valves open on "overlap"). Even with valve adjusters removed, the rockers on the exhaust/intake cylinder will be tight against the valve cap.
  11. timbeck

    timbeck XS650 Junkie XS650.com Supporter

    If you run a PMA it is a good idea to put a dot of white paint on the rotor, the nut and the threads at TDC. That way a quick glance you can tell if the rotor has slipped on the shaft because you are not using a woodruff key.

    wannabridin and gggGary like this.
  12. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    This is true Griz BUT both pistons are at TDC at that point, and neither one hits the valves, so they are not open enough to strike the piston, even with tappets set where they should be, IF the cam is set at the "TDC" position. This is indicated by the hole for the pin being either straight up OR straight down depending on which cylinder is on the compression stroke.
    GLJ likes this.
  13. Downeaster

    Downeaster Everything in XS Top Contributor

    Poop. I knew that, just wasn't using both brain cells. Thanks for cleaning up after me...
    GLJ likes this.
  14. 5twins

    5twins XS650 Guru Top Contributor

    But ..... I have found you can loosen the adjusters up enough so none touch a valve. That's how I disassemble and assemble the topends. I don't want a rocker trying to push a valve open while I'm removing or installing the top cover.
  15. wannabridin

    wannabridin XS650 Guru

    Great info guys, thanks for the help. When I find some spare time to devote to it, i'll get everything tested out.

    I am running a PMA and had not done TDC checks prior to this. so i'll need to verify all this beforehand. Fingers crossed i can just skip the chain a little more and get it back into position. Otherwise, out she comes for a budget top end rebuild (which might not be the worst thing)
    TwoManyXS1Bs, gggGary and timbeck like this.

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