Timing chain came off, how much work can I expect to do?  By  florida (Read 7,792 times)

again, that would NOT cause the chain to fall off.

I suggest you look for another cause.

Good catch.

.004" is actually good. That is fine, and should not cause any problems. Set your intake at .004", and the exhaust at .005", and that is good.

There is the cause of the derailment,,,is that part of the chain tensioner? Or part of the funky anti kick back device that sits up by the cam area?

I'm going to bring it to the Local retailer of this brand scooter. They are knowledgable and might be able to identify the metal piece.

I'm certain that the broken piece of metal caused the chain to jump off the cam. One link of the chain had been stressed enough that I had to flex it laterally so it would loosen up and become unkinked.

I think it's from the bearing cage on the variator side of the crankshaft. The "rivets" are the only reason I think that.

I've never seen a cam stopper/holder in person, although I've seen them before on forums and after searching for "gy6 cam stopper" it doesn't look like those have rivets at all. Do 139qmb engines even have cam stoppers? I can't say I've ever seen one on a 50 (in forums), but who knows things change. I always thought those were only on 125+cc engines.

Florida--- was your scooter engine running louder than normal the last week or two before the chain came off?

Yes, so as you have been told this is the remains of the cam stopper - which prevents the engine from rotating backwards.  it somehow came loose, got between the cam and chain, and popped the chain off.

there are now several questions you need to answer for yourself: 

1,  chain.  i'd guess there is a 75% chance your chain is OK - but that means a 25% chance it isn't and will fail.  on an overhead cam, negative clearance engine like the gy6, if the chain breaks you are almost guaranteed to have either bent valves, a hole punched in the piston, or both.  you also have the distinct chance of metal fragments in the oil that then cause extensive damage.  on the other hand, replacing the chain is a total engine overhaul - removing the jug, separating the halves of the crankcase and taking the crankshaft out just to put a new chain on.  so, basically a total engine overhaul.  so you need to weigh in your own mind these 2 and come to a decision.

2.  cam follower:  of course, it should not have come loose.  usually the 2 cam dowels go through holes in the follower,  did both dowels come loose?  you may want to get a new head just to be sure since they are so cheap.

3.  oil at head gasket:  the oil leak may simply be from a dry valve cover gasket or loose bolts.  most likely the oil then dripped down, some staying at the small lip of the head gasket and appearing to have come from that location.  a good cleaning would resolve this.  but maybe (long shot, but possible) the cam follower being tossed around somehow caused a leak at the head gasket (higher likelihood it might have scratched the cylinder below the piston - did it smoke at the end?).  so, best to keep a close eye on this.