Timing chain tensioner question


'99YZ400 bought used in 2005, rode for 2 years with nothing done but

oil changes.

This spring, decided to get "responsible" and check valve clearance.

3 or 4 were tight, bought new shims, installed, clearance was now good, reassembled, started ok, took quick spin, ran fine.

Fast forward 5 wks, trip to trails planned on Sat. AM

Friday PM, getting ready for trip, started bike to warm up to check oil.

Started ok, rev'd a few times, stalled suddenly with horrible crunching sound.

Tried to kick, unable to turn over fully.

Took off valve cover, timing totally out of wack, chain obviously slipped. Reset timing, now able to kick but no compression at all. Valves suspected bent.

Removed head, all intake valves bent.

Took head, w/ cams into dealer for rebuild. They replaced all 5 valves and now I'm ready to re-assemble....but, my question is what went wrong the first time???

During the disassembly when I re-shimmed I backed the chain tensioner all the way out until it clicked/held at the end, removing all the chin tension. After the re-shimming, I turned the tensioner half a turn and the spring took it the rest of the way, and I assumed the chain was re-tensioned.

After the catastrophic failure I removed the tensioner and it seems to work fine. The only thing I can think of is maybe the spring inside the tensioner has gotten tired.

Does anyone have any clue as to why the chain jumped the gears after I re-shimmed? I'm very concerned about reassembling the head after I spent $300 getting valves fixed.


Removed hea


During the disassembly when I re-shimmed I backed the chain tensioner all the way out until it clicked/held at the end, removing all the chin tension. After the re-shimming, I turned the tensioner half a turn and the spring took it the rest of the way, and I assumed the chain was re-tensioned.

Did you have the timing adjusted properly? Did you take up the slack on the chain toward the front of the engine prior to releasing the tensioner?

Have you ever replaced the timing chain? If your bike has many hours on it and never had the timing chain replaced, it's possible that it jumped time.

I have heard of the gear on the cam it self spinning just a little but still looking fine. It's just another thing to take a look at.

If your timing chain stretches beyond a point, it'll become too long for the tensioner to keep proper tension on it (tensioner only goes so far until it hits it's own limit). And although the chain looks perfectly good, it's not until you take it out and lay it next to a new on when you realize the thing is 2" longer than the new one.

Since it sounds like you're putting it back together - REPLACE the chain - a little more work, but SO worth it.

4 possibilities come to mind:

> Worn chain

> Worn crank sprocket

> Faulty tensioner

> Seized camshaft

If you failed to torque the caps correctly, cam seizure is the likely cause. The spring in the tensioner does control how much tension theh chain gets, but it does not hold the tension directly. It simply rotates what amounts to a nut on what amounts to a bolt any time the tension is low enough to allow it to. The chain cannot push the tensioner back in any more than you can make a nut rotate by pushing on a bolt.

Thanks to all for the replies.

Some answers to questions:

I'm convinced it was timed fine on reassembly for two reasons. It ran fine when I first started it, and I took pictures of everything before dissasembly and they looked the same when I initially re-assembled.

I did not replace the chain (yet), but based on the answers I will. It will be interesting to see the length difference.

BASSic asked:

Did you take up the slack on the chain toward the front of the engine prior to releasing the tensioner?

I'm not exactly sure what that means, I assume you mean make sure it the chain is tight on the front side so that all the slack is on the back side where the tensioner is. If so, then I can't honestly remember doing that...could be where I failed. Since the chain rides on three gears and the tensioner can only remove tension from the backside then if I left too much slack on the front side it could slip. That could be the culprit right there. I don't recall making a concious effort at checking that but I sure will this time..:excuseme:

I think the cam siezure is low probability, as I used a good torque wrench, patterned the tightening.

Anyway thanks to all and hopefully with any luck I'll get it running this time around.

No one thinks I should get a new tensioner spring?

If yosu failed to run the slack out of the chain, it would only have made a difference if you had turned the engine backward first. As soon as the crank pulled the front run tight, the slack that developed at the rear run would have been taken up immediately. And by that, I mean immediately.

It wouldn't hurt to replace the tensioner. There are only about three things that can go wrong with it, though.

> It can become contaminated with debris, or otherwise caused not to operate smoothly. You should be able to wind it up smoothly, and feel it unwind smoothly.

> The spring could break, become detached, or become weak. The spring should unwind assertively. Some of the newer ones have been found to be very weak.

> It can wear the internal "threads" to the point where they slip under a load. You should be unable to push the plunger in from any position without rotating the winder.

Grey is right on with the slack - but what that would also mean - (just to add to his comment) is that at this particular instance, your timing would be off immediately because the crank moves, takes up the front slack, THEN the cams start to move.

So, your bike would immediately run like crap with the timing being off.

From my experience with these, there really isn't an easy way to actually get slack at the front side - by the time the cams are in, it's all pretty tight even before putting in the tensioner. Now if your chain is extremely stretched it might be possible.

For what it costs - I would put in a new tensioner, it's cheap insurance that everything is up to snuff.

Since you have photos you should be able to compare your cam gear position THEN to NOW - if the gear is exactly where it was before then your gear did not slip.

Cam seizure usually leaves visual traces on the cam and/or journals as well, so you should be able to tell if that's what happened.

It's always best to turn the engine over 10-20 times before putting the lid on and setting TDC again and verifying your cams. I think it even stated this in the manual.

Also - set your torque value just a tiny bit lower that specified for the caps.

Hello Extrapolate, I have a 99YZ400f also and pretty much the same thing happened to my bike, although I haven't pulled the cap off and checked shims, timing, valves Etc.

I noticed an oil leak and loss of power while out for a ride so I gently nursed it home where it sat for 2-4 days, then my partner "accidentally" knocked it over in the shed (she was pissed at me cos we'd had an argument) n being the size of a pigmy possum she was unable to pick it up until I got home about an hour later, I then services the bike changed oil cleaned filter etc and cleaned the carburettor, once reassembled I tried to kick it over and almost broke my leg. It felt like the engine had completely seized or locked up, the lock up freed up after a good hard kick as the first kick was a bit girly, but when I kicked it made kind of a crunch noise and now when the kick starter is going up it makes a higher pitched cracking sort of noise similar too some 2 strokes I've heard, can some one please help me

basically what I want to know is what ended up being the problem and what was the solution, grayracer I messaged you then posted about this a few weeks ago, but to my disappointment you did not comment... Do you think something similar could have happened.

You know this is a 7 year-old thread?


Falling over with the engine shut off won't make it skip time.

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