Uh oh, jumped cam chain???

wannabridin

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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??
 

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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.
 
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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.
 
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.
 
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.
 
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.

True. I'd forgotten about that. Thanks for the reminder.
 
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.

Also true and worth a try. I tend to just go for the worst case scenario sometimes.
 
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.
 
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.

tim
 
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.
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.
 
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.

Poop. I knew that, just wasn't using both brain cells. Thanks for cleaning up after me...
 
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.
 
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)
 
Ok, rigged up a degree wheel, used the piston stop to find proper TDC, set up a pointer and started taking measurements (on the left cylinder) as best I could. I had to hand turn the flywheel as I couldn't get the degree wheel rigged up otherwise and I can't find the flywheel puller. Getting consistently accurate measurements were difficult. I've a single dial indicator, but getting the mag base and extensions to work well wasn't in the cards for tonight. Here are my results though. I believe that I'm close enough to consider the cam and crank back in alignment and my TDC mark to be valid. What do the experts think???

upload_2020-4-21_0-15-33.png
 
Re-hashing my work last night, I believe 10 degrees or less is well within the acceptable range of observation. It's very difficult to tell exactly when the valve starts to move by eye w/o the dial indicator. I suppose I could borrow a 2nd indicator and base and rig up an attachment for both valves and get more accurate (watch for when the needle just starts to move and finishes moving) to hone in the accuracy.

When I remove the outliers (based off 1 standard deviation), then I get more accurate data. I believe the intake opening has to do with how the flywheel moved in my hands and the lighting differences. I believe it's even closer to to other values based on these variables/interpretations.

upload_2020-4-21_10-45-25.png



Any thoughts??
 
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