Dealer claims it has a broken timing chain. I doubt it.

davidc83

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Whoa, the whole thing could boil down to a bad spark plug wire....
Or plug I guess (broken internally, if that ever happens).
old trick used to do in the 70s when spark plug wires were suspected of being bad....wait until night fall.....make sure no light anywhere and start the vehicle, open hood, bad plug wires had a tendency of 'jumping' spark and you could see the little dances of fire....
 

jtanner

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Alright gentleman, I'm locked in on the issue.

I put fresh plugs in and started it up again. No change. I ran it for a solid minute to see if the left plug would be wet again. It wasn't. I repeated this sequence with the same results. Must have been a fluke the first time with the wet plug.

Anyway, I'm glad @670cc suggested I take a look at the timing because that was the next place I looked. Up to this point I assumed that if the engine wasn't timed right then it wouldn't run at all. I was wrong there. The cam is clearly timed one chain link behind the crank (see pictures). To make sure I wasn't reading the marks incorrectly, I checked my good NC to find that everything was lined up perfectly as I would expect.

I'm relieved to have found the issue, but now I'm wondering how the heck it happened.

I haven't done this type of work on a twin-cylinder bike before. I know on a dirt bike all you need to do is release the cam chain tensioner, unbolt the cam sprocket from the cam, clock it to the right orientation, then bolt it back on. Sound about right? Wish me luck :D
 

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670cc

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Good to hear you found an issue that could explain the symptoms. My foremost thought at this point is that I hope you have a service manual for the NC.

As you already know, if the cam chain jumped a tooth, that strongly sugggests a cam chain tensioner issue. There have been a few tensioner issues brought up on the forum in the past. Perhaps one of those old threads could shed some light. Here is one: https://www.nc700-forum.com/threads/cam-chain-tensioner-failure.7333/
 

jtanner

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Yep, it was a tensioner issue.

Looks like that little press-fit pin at the top came loose and seized it up. Damage is evident on the chain with some spots worse than others. The chain damage has me scratching my head because it makes sense that the links in contact with the sprockets at the moment of slipping would take a beating, but it appears that every single link has similar indentations. What's up with that?
 

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jtanner

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Holy smokes... I just realized there's a whole spring missing on the tensioner. It's completely gone. What in the world?
 

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lootzyan

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You know it's not missing. It's there and you have to find it. You can't risk a loose spring in the engine. If it hasn't fallen into the oil pan, you can try to fish it out with a magnetic pick-up tool. Good luck.
 

lootzyan

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Is the spring hook on one side broken off? Check the oil pan drain plug.
 

jtanner

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Is the spring hook on one side broken off? Check the oil pan drain plug.
Yeah that hook on the one side is missing. I fished for awhile with no luck, and it didn't come out when I drained the oil out of the drain plug yesterday (at least in one piece that I could see). There were some metal shavings in the oil I drained. Not much, but that could possibly be the source.
 

lootzyan

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...Not much, but that could possibly be the source...
I don't think so. The metal shavings most likely come from the cylinder rings and bearings, which is quite normal. This missing spring hook is too large to get in between moving parts. And it's too small and light to be broken into smaller pieces. In addition, the oil acts as a cushion. So if it was not caught by the drain plug magnet, so when the engine is not running it will be at the bottom of the oil pan. You can drain the oil a few times, filter it and use again without fear. Alternatively, someday you can remove the oil pan.
 

jtanner

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Solid advice, makes sense! I really appreciate your input.
 

MZ5

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Seems to me the source of the shavings is likely the damaged timing chain links, and maybe the guide or tensioner.

IMO&E, if you’re getting enough metal off the rings and cylinders to be able to easily see with the naked eye, your engine has a problem.
 

SlimDude

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So in the end it seems the bike shop was correct in the source of the problem but not necessarily the solution if the chain is still safely functional?
 

jtanner

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So in the end it seems the bike shop was correct in the source of the problem but not necessarily the solution if the chain is still safely functional?
Yeah I suppose you could say that. I'm just curious how deep they investigated the issue and how they handled the conversation with the owner. They told him that replacing the engine was the right step and quoted him the price for that. From what I can tell, a fresh cam chain and tensioner will likely clear the issue up. We'll see though. Parts are in the mail, I'll keep you updated.
 

jb882

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Yeah I suppose you could say that. I'm just curious how deep they investigated the issue and how they handled the conversation with the owner. They told him that replacing the engine was the right step and quoted him the price for that. From what I can tell, a fresh cam chain and tensioner will likely clear the issue up. We'll see though. Parts are in the mail, I'll keep you updated.
I doubt they went into it very deep at all. Seems like the default position these days is replace instead of troubleshoot, find the root cause and repair. Its great you found it and should be able to get it up and going. I kind of feel bad for the prior owner, he should have been able to get the bike fixed easily.

Not bike related but my sister went thru a very similar scenario with my nephew's truck that i gave him when he got his license. It would randomly stop running and the dealer she brought it too after about an hour of troubleshooting told her it needed a new motor at a cost of 8k. Blamed the "cam bearings were tightening up causing the motor to stall" She called me for advice and i told her i did not believe for a second that it was a motor. The Silverado with a 5.3l is pretty darn near bulletproof.... I told her to tow it to my house and i would look into it and worse case bring it to a mechanic i trust. It ended up being a bad ground wire for the coils which is a common issue on those 99-06 trucks that i found on a quick google search. It would sometimes lose the ground when he hit bumps and the truck would stall. I traced the ground and found the issue in about 20 min, fixed it and 3 years and 30k miles later he is still driving it.
 

thanosgp

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could someone have done a botched valve adjustement on the false TDC? If you know the engine is entirely adjusted correctly to spec, then you can approach the problem as a part failure.
Been there,done that due to false description in downloaded manual.Sounded like a tractor...
 
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