01 YZ426 - Intake cam seized while bike wasn't running!?

So I've got a problem here and from the research I've done so far I'm trying to determine what would cause this issue and what's the best course of action now. I'll try to keep it short, but I tend to over explain everything to make sure I'm not missing anything.


Anyway, I've got an 01 yz426 that I've had for 5 years or so now that I've pretty much gone top to bottom repairing/replacing things myself over the years while only putting 60 hours on it (mostly all trail riding). I did the 03 auto-decomp cam right when I first got the bike and shimmed the exhaust valves accordingly. Checked the intakes at the time and they were still in spec so I just left them alone. Anyway recently it was having issues idling after it would get hot so I started with rebuilding the carb and new plug. Everything ran awesome and I went riding for a few hours and the problem seemed to go away. 


Fast forward to last Friday. Pull bike out to go camping, check everything over, fire her up as I usually do for peace of mind before I take off and shut her off like normal. Now here comes the interesting part, it ran absolutely fine and I shut it off myself with the kill switch. Drive several hours and go to fire it up after setting up camp and it would kick to a certain point and then stop. I immediately assumed the worst and tried turning it over with a socket as to not mess anything up any further and I could only get one revolution back and forth before it would stop and sound like the piston is hitting the valves. OK, &%$#@!?! Load everything back up and try to have a good time knowing what's in store when I get home.


Came back, did some homework on what I've got to look forward to, and start to tear it open. First thing I noticed was the timing was off causing the intake valves to stay open when around the TDC vicinity. Ok, so I assume somehow the timing chain skipped and bent valves. Check the valve clearances though and the exhaust side is still where I set it 5 years ago, but the intake side was super tight on the right 2 valves (clutch side).


E .25mm .25mm


I .12mm .05mm .08mm


Before I measured the clearances though I ended up taking the cams out to verify the motor would turn over more then once (which it did). First thing I partially noticed was the intake cam stayed in the cap when I took it out (not really thinking anything of it at the time), the bolts were on much tighter then the exhaust bolts I properly torqued years ago, and immediately noticed the right 2 cam journals were wallowed down a bit (doesn't look like the buckets will come out without some work). Go to put them back in to measure clearances and notice the exhaust cam spins easily while the intake cam won't turn without using something to turn it back and forth. Take it back off again and that's when I fully noticed the cam cap is warped causing the cam to get jammed in it. Obviously someone did this before I owned the bike and torqued them down way too much.


So how the heck did my bike go from running and idling fine to this after just shutting it off? I understand the over torqued cam cap and low valve clearance being a factor. I can also understand if this happened WOT on the trail, or hell even it if was running at all. What I can't figure out is how after shutting my bike off this one time my cam seizes while it's not running while also simultaneously my timing chain being loose enough that it would be able to skip when trying to kick it over? Or is that just what I've been dealt this time around and should be happy I got 60 hours out of it with over torqued bolts, rebuild it, and move on?


If that's the case I've done plenty of reading recommending Engine Dynamics in Petaluma, but what should I expect to spend for them to fix it? I would assume that depends on how bad the damage is and whether the cam cap is usable as well as the cam itself. And what else should I look at in terms of inspection and/or replacement other then the timing chain/tensioner/guides and the obvious gaskets? Going to pull the head off today to verify 100% there is no damage to the valves. Thanks in advance for any help and I'll try to post of some pictures later today.

Edited by ladmozx

EDCo charges around $250-350 to fix cam bores on YZF's.


I have a dollar sitting here that says that if you pull your cam chain out and hang it on a hook, it will have a bunch of kinks in it, instead of freely hanging in a nice straight loop.  A kinked, stiff-linked, binding chain is probably the cause of your problem, and that being the case, it's most likely to show up on shut down.  The kinks in the chain keep the slack rear side of the chain artificially tightened while running, "fooling" the chain tensioner.  The engine stops, bounces backward off the compression stroke, and the reverse rotation causes the tight slack sides to switch places with each other.  The crank pulls all the kinks out on the back side, leaving the front suddenly slackened, and no tensioner on the front to deal with it. Voilà, the chain slides over a handful of teeth.  You just don't know it until you try to restart.


If your chain is like that, it happened because your oil either is not doing the job for you, or you aren't changing it often enough.

As always grayracer comes through with an answer  :thumbsup:


I had to order a flywheel puller so I haven't been able to pull the chain yet, but looking at it down the cover it appears to move fluidly with no binding. However following the same theory as the kinks in the chain, my seizing intake cam would have performed almost the same action as the motor bounces off the compression stroke allowing the chain to skip over the intake cam. I inspected the cam cover and can see some marks on it where the slack in the chain has slapped it a few times.


I called EDCo and talked to Jenny (sorry if that's wrong) and she said it's very common for the yz's to burn out the intake cam in general, but oil starvation or over torquing of the cam bolts is definitely the most common. Considering I changed my oil every 3-5 hours and always checked my oil level every ride I'm going to say it was probably the torque issue. Unless 10w-30 Castrol GTX is crap in which case that's my bad and deserve this.


Just so everyone knows EDCo currently charges $175 for a single side and $318 for both and depending on the condition of your cam they can clean it up for no additional charge. Not as bad as I was expecting especially if they can fix my current cam and I don't have to replace that as well. There are no score marks in it so hopefully my luck runs better there.


Question though. Looking at things currently are you able to pull the head while the motor is still in the bike? It looks like short of pulling the timing chain guides out there isn't enough clearance between the head and the frame to pull it off with them on there. I'm not opposed to pulling the motor, but if all I have to do it wait 2 days for the flywheel puller I'd much rather do it that way since it's not going anywhere any time soon.

Edited by ladmozx

Remove the flywheel and stator, remove the tensioner, then unbolt the pivoting rear guide from the crankcase, rotate it 90 degrees on the vertical, and pull it down and out.  Unbolt the head and raise it off the cylinder.  Pull the front guide up out of its seats and lift it out of the way.  Then you're clear.  Assemble in reverse.

As mentioned, EDco charges 175. They machined my intake journals, polished my cam cap and my intake camshaft for 175 and it was returned to me within 48 hours. Could not have been happier. No shipping was involved, as I do live a few miles from them. Just a note

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