YZ426 Bent Intake Valves


6 replies to this topic
  • MNellis

Posted December 12, 2004 - 09:56 PM

#1

As some of you may know, I just purchased a used '00 426 that appeared to be in great condition. When I picked the bike up it started easy enough, didn't smoke and generally seemed to run great. I know the owner and the bike has only about 50 documented miles on it. The owner uses a trip computer to document the hours ridden as well as the miles.

Anyway, I took it out after picking it up for a nice muddy ride. It ran great for about an hour then my buddy mentioned it was puffing a little oil. I thought that strange but continued to ride. A little while later while on a trail the top end seemed to be making more noise and it continued to get worse. We stopped for a water hazard and when I let the clutch out it died. I tried to start it but it wouldn't even kick over...obviously locked up.

My first thought was that it dropped a valve. I finally got it apart this evening and my fears were substantiated. It didn't actually break a valve but all three intake valves were bent and there was a slight mark where one of the valves touched a piston (see link). I'll take it in and have the shop check the guides and replace the valves but my question to the group is ......

is this a common problem? Since it was running fine prior to the abrupt stoppage it would seem unlikely that the cam chain skipped a tooth. The excess noise prior to stopping suggests a cam chain problem but why the oil burning (which was fairly heavy prior to stopping)? The cams look to be in good condition and I was going to do the 450 cam mod anyway.

Could I have over-reved it and floated the valves? Does the 426 have a rev limiter built in? I didn't think I was over reving it but then the trails were narrow and I never got it out of 3 gear with most of the day spent in 1st and 2nd so I suppose that's a possibility.

Thoughts??
Bent valves here

  • yz426onNOS

Posted December 13, 2004 - 03:27 AM

#2

This is one of 2 major problems with the 00 426. Mine had the same problem but worse. I dropped a valve it snapped at the weld point. They are shiza 2 pieace valves. One of the reasons the 01S and up came with titanium one pieace is to solve this problem. Just get the valve seats and guides replaced. Throw a new piston in there with some new rings. I recommend the 13.5.1 450 bore wiesco. You will be very happy. I prefer the hotcams over the yamaha cam. You will get the auto decompression and a Lil more power.

You did not do this by over revving the bike. It happened because of the 2 pieace valves, there crap. Get some one pieace kibble white valves and you will never have this problem again!

the second problem is the counter balancer drive gear. It has a woodruff key which shears. You will notice it as a clanking sound at idle behind the water pump. It will go away when you rev it. Mine lasted 5 years before it went bad. Thats with me beating the living crap out of that bike twice a weekend for five years. You just have to make your own stronger woodruff key out of some woodruff key stock. I believe it is 5mm. Or you can buy the 01 assembly!

If you have any other questions feel free to ask. :cry:

  • sirthumpalot

Posted December 13, 2004 - 03:30 AM

#3

There is a rev limiter and these things do not typically fall apart from being revved to the limiter for a short period of time. Sounds like something else. When you took it apart, did you check the cam timing? I ask because there are 2 things that could have happened. Most likely is the cam chain skipped a tooth somewhere, but that's most likely on higher hour motors or motors that did not get frequent oil changes (grill your buddy about his maintenance). Second the cam sprocket may have spun on the cam. You'll know this if the timing marks line up, but one of the cams is obviously in the wrong position. Let us know what you find!

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  • MNellis

Posted December 13, 2004 - 05:28 AM

#4

When you took it apart, did you check the cam timing? ...... Most likely is the cam chain skipped a tooth somewhere, but that's most likely on higher hour motors or motors that did not get frequent oil changes (grill your buddy about his maintenance). Second the cam sprocket may have spun on the cam. You'll know this if the timing marks line up, but one of the cams is obviously in the wrong position. Let us know what you find!


I wasn't able to check the cam timing because the motor was locked up. I'll check the cam sprocket for any obvious signs of a problem also.

This is one of 2 major problems with the 00 426.... They are shiza 2 pieace valves. One of the reasons the 01S and up came with titanium one pieace is to solve this problem. Just get the valve seats and guides replaced. Throw a new piston in there with some new rings. ...
You did not do this by over revving the bike. It happened because of the 2 pieace valves, there crap. Get some one pieace kibble white valves and you will never have this problem again!

the second problem is the counter balancer drive gear. It has a woodruff key which shears. You will notice it as a clanking sound at idle behind the water pump. It will go away when you rev it. Mine lasted 5 years before it went bad. Thats with me beating the living crap out of that bike twice a weekend for five years. You just have to make your own stronger woodruff key out of some woodruff key stock. I believe it is 5mm. Or you can buy the 01 assembly!


Thanks for the information re: the 2 piece valves. I guess I didn't realize that. At this time, I don't plan on changing the piston out but who knows, after some time in the saddle to get my "dirt legs" back I may have to. Thanks also for the infor re: the rev limiter. I didn't think I was being overly agressive on the first outing either.

I'll be sure to let you all know what I find out.


  • grayracer513

Posted December 13, 2004 - 01:18 PM

#5

This looks like either cam timing, or a loose big end, and here's why I say that. First, if it had anything to do with the valves being made from two-pieces, the issue would be that one or more of them broke at the joint between stem and valve head. Since they aren't, I would rule that out.

Second, if it were due to valve float, it's not very probable that all three would be uniformly damaged as they are. Since a healthy 426 doesn't float the valves at the rev limit, you would need to have one or more worn and weakened valve springs, and then you wouldn't have all 3 looking so much alike. You could have done a mechanical over-rev by downshifting the engine into an RPM range beyond 11,000, but I'd guess the results would still have been less uniform.

You also said it made a lot of noise before it quit. That could have been all the suddenly acquired valve clearance, but you need to check the big end rod bearing for clearance. If you've had a rod bearing failure, the piston could have come up and smacked the valves because of that. Notice how clean the areas at the outside edge of the combustion chamber are, as if the piston may have hit, or nearly hit it.

You might want to check the condition of your crankshaft cam chain sprocket to see if it's excessively worn. Of course it shouldn't be, if the represented time on the engine is accurate. But, if cam timing is going to be your prime suspect, it won't hurt.

Good luck!

  • MNellis

Posted December 13, 2004 - 01:48 PM

#6

This looks like either cam timing, or a loose big end, and here's why I say that. .......... That could have been all the suddenly acquired valve clearance, but you need to check the big end rod bearing for clearance. If you've had a rod bearing failure, the piston could have come up and smacked the valves because of that.

You might want to check the condition of your crankshaft cam chain sprocket to see if it's excessively worn. Of course it shouldn't be, if the represented time on the engine is accurate. But, if cam timing is going to be your prime suspect, it won't hurt.


Thanks Grayracer. You make some good points and I hadn't thought about the big end going bad. I pulled the head with the engine still in the chassis but I went out and fiddled with the piston a few minutes ago. I'll have to pull the cylinder to check it further but it feels as thought the piston will rotate a little in the bore (radially). I don't know how much is normal but it looks like I'll have ot pull the cylinder to find out.

I agree with you though, I think it's a cam timing problem...now I just have to figure out what went out of wack.

Thanks for the thoughts.

  • Satch0922

Posted December 13, 2004 - 07:44 PM

#7

maybe I missed it but did you check the cam chain tensioner? That can cause the failure you describe.





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