YZ400 knocking noise! think ive found the problem...

Ok, what i have is a yz400 99 model.

Here is a short background to what happened before i go on with teh story and ask for some help.

A few of us went for a ride, i droped the bike in the river and it was submerged a fair bit. Motor was still running went it came off on me and i was pushed of and didnt have a chance at getting it switch off, so it stalled by itself(guessing water went in?).

Anyway, after pushing it to dry land, and getting rid of all teh water, upon starting the bike, it started making very laud noise, very similar to rod knock, or piston slap etc.. mates told me most likely piston or rings gone, just get a rebuild kit and replace it, so that is exactly what ive done.

Bought a wisco oem spec piston kit, and while at it, checked some freeplay on my rod and it seemed fine.

Anyway, i ended up getting it all in and finished.

Started teh bike, and voila! all good, no noises or anything, thought i fixed it and that was it.

After a few weeks, mate asked to go for a ride and i thought id start teh bike again, check oil and just overall look over it. Upon starting teh bike, within 2-3min, BANG...a few little noises and oops, thats SAME exact noise i had before came back!!!

I was devestated, couldnt figure it out! and so i thought must be big end/rod noise.

So i started searching the forum, and found some issues with a crank balancer keyway that was talked about? so i looked at mine and here is what ive found...


I am guessing these are teh gears that are talked about when saying balancer shaft and crank shaft gears?


Upon further inspection, ive realised i have no keyway on my balance shaft but just splines! so cant be that, right?



So next i looked at teh crank shaft spline, and that had a keyway! see picture^^^

Trying to figure out if this is what you guys were talking about, and went ahead and removed teh shaft and tried to see how much play it has... well, it has very minimul play in it rocking it left to right, im guessing this is ok???

How much play should i have here?

Then i thought! why not check my timing... well, lucky i did, have a look at teh pictures...


So i timed it right?


As you can see by where the dot is on teh gear, you can see its not lined up perfectly with the cylinder head! seems to be one tooth out on teh exhaust side? am i right?


Next is the intake side, same thing, out by one tooth or so by the looks of things!

So, conclusion that im getting to is, is this where my knocking noise would be coming from? i mean, if its advanced too much or what not, it would create wrong timing/firing and would create such a bang/knock! im hoping its that anyway...

Question next is, why did it jump on me? obviously, it was running good until it jumped after a few minutes after starting it and thats when the noise came about.

Now i just have to check the chain and the chain tensioner.. can anyone tell me how to check the tensioner please?

I mean, i had it removed before and by just looking at it, i couldnt see anything wrong, how much force/tension should it have?

I just hope i havent done any valve damage, but ive rotated teh engine by hand and nothing hits, so its a good sign.

Please give me some opinions and specs to look for now, and any help is appreciated greatly, sorry for the long read, hope this helps someone out one day :worthy:



EDIT: ive just realised i have not timed it properly, as ive noticed teh marking on teh timing gears ( exhaust- E, and intake-I) are 180deg out, so i shoudl have turned teh crank one more revolution! i wonder if teh markings are parrallel to each other? off to rectify and recheck this.

EDIT2: Ok, went and turned teh crank shaft onather one revolution, and the dotted markings still remain the same, the intake cam is one tooth out easy.

Also noticed, my tensioner isnt doing much at all, i was able to install the tensioner by hand all the way into teh hole, without turning that screw on teh inside of it so the rod can be pushed back inside, wich means, there is stuff all tension on the chain bracket!

Possible broken spring inside the tensioner? I had my timing jump once on my 426 but never happened again. I have heard this can happen sometimes. I believe mine was only on intake though, so when it happened it wouldn't start again, no compression.

Common causes for the chain to hop a tooth are if the chain is too loose (bad tensioner, stretched chain, etc..) and/or if the bottom (crank) sprocket is worn. If it were me I would in the least replace the chain, and give the tensioner and lower sprocket a serious looking at while it's apart. There's others here with more experience in the motor than me so hopefully someone will chime in with more advice.

Thanks for the reply guys.

Ive had a look in teh manual, and there is nothing to tell me how to measure if the timing chain is within specs!!! can anyone tell me how i can measure it?

Does anyone know if the chain can be replaced without removing teh stator/sprocket at the crank? i had a look, and there is no links on teh chain to remove or anything like that, and it seems like i will need some kind of puller to remove the sprocket/gear where the crank and stator are?

Anyway, here are a few more pics.


Thats the tensioner under full extension, about 23 to 24mm, again, there is nothing in teh manual that can tell me how to check the tensioner!



Ive rotated teh crank until the chain was at its losest point, then pulled up and down on teh chain to give you an idea of how much slack there is, this is with the tensioner still on!


Cam gears look to be good, can anyone see any different?

I will be getting some quotes on monday for the chain and tensioner.


As already mentioned, there have been a few reports where the gear down at the crank has simply worn out. This would give you the potential for jumping a tooth.

Very possible if the bike has a lot of hours.......

If you have that much slack with the cam chain tensioner fully tight then your chain has stretched,replace your tensioner,chain,maybe crank and inspect your cam gear on your crank. They do wear on the crank like what was said before. I had a 450 that was just over a year old and you could see it was worn alot. They bad part is that when that gear wears out you have to replace the crank. That gear is welded on the crank. One thing I wanted to say I have seen guys be off by one tooth on the cam gears because they forget about when you get it all together then tighten the cam chain sometimes it moves just one tooth. So maybe this happened when you put it back together?

Also I dont see the nut on the counter blancer nut infront of the gear did you take that off before the pic? just wondering


Like someone else said, if you can get that much slack from your chain then either it's worn out or your tensioner is bad.

We replace a lot of YZF cam chains at my shop as they do seem to stretch fairly quickly. Once they stretch it will be hard to get the cam timing marks to line up right. It will look like they are 1 tooth off. You can tell if the chain is bad when you take 1 of the cams out, the chain will have kinks in some of the links.

Also when you have the tensioner out, look at the arm when its all the way extended and see if you can see scrape marks from it slipping from the locking mechanism letting loose while engine running.

If the chain needs replaced you will have to remove the stator assy to get the chain out. The lower sprocket is part of the crank so hopefully it is still good otherwise the crank will need replaced.



well, for anyone that is interested, i have put it all together and its running fine again.

The problem was: teh timing chain!

I have replaced teh chain and straight away, the slack was gone and the timing marks were spot on...

Also, the knocking noise has dissapeared, wich made such a bad noise i would of thought teh big end or rod/piston was gone.

Anyway, all good now and hopefully someone will find this thread usefull in teh future if something similar goes wrong with they'res



yea the timing chain was an issue on a lot of yz400's once they got around 4-5 years on them. I replaced mine as well.

I had a strange issue with mine where I spun the gear on the intake cam. You could time it like you were supposed to but the actual cam lobes wern't in the right place. When its timed correct you should just about be able to put a straight edge across the tops of the intake and exhaust cam lobes. The intake and exhaust lobes should be pointing away from each other about 15deg up from horizontal. Took me forever to figure out that the gear had spun. The gears are just pressed on. No splines or keyways. So if the motor stops suddenly (like a hydrolock) the gear is usualy what gives. I had the gear tack welded on my cams. No more spinning gears for me.

You must have a very good camera! Good luck on the repair. Sounds like you need a chain and tensioner to me.

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