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Timing Chain Jump at Crankshaft

D

DaveP

Forum Mentor
This is a continuation of my thread at http://www.thegsresources.com/_forum/showthread.php?245453-GS650L-Intake-valve-not-seating about the loss of compression in the #3 cylinder and sudden poor running of the engine. I took a look at the timing tonight, not good. The timing jumped and most likely bent at least this one valve enough to prevent it from closing.

The two cams are still aligned and the correct number of chain pins between the timing marks, but the crankshaft is not at TDC any more... the cams are ahead of the crankshaft.

Scenario on the road when this problem occurred: back road, traveling about 35 mph around a somewhat blind curve and a car pulled out in front of me. Not quite an emergency stop, because I was able to hit the horn while rapidly slowing down behind this car, but that was the end of the good running engine. I was downshifting quickly, and braking hard, but would that have caused the chain to jump on the crankshaft?

Pictures will be added in following posts since I can't add more than one at a time.

Dave
 

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Seems the problem was the tentioner stuck or has broke. And yes the chain could jump at the crank as slack.... or whip if you will..happened. Time to yank the head and get it right again would be my approach.
 
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On at least half the cam chain tensioners I've encountered, someone has tightened the locking screw against the pushrod, therefore ensuring that the tensioner can't work.

http://bwringer.com/gs/camchaintens.html

No idea whether this is a partial explanation for what happened to your bike, but definitely pay attention to the state of the tensioner when you remove it.
 
The previous owner had the bike for 30 years. I don't think he did anything except for oil changes and tires, so everything else is just age and neglect.

Thanks for the link on the tensioner re-build. Your site is invaluable.

Dave
 
Pulled off the head today. The chain guide for the cam tensioner is snapped in half right where the tensioner would press against it.

IMG_5042.jpg

Does not look like much availability for this part...
 
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