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MiGkillr

Thumper Stumper - of valves and things

8 posts in this topic

I've been lurking a lot and doing a lot of reading. I'll be brief, but could use some advice.

The bike is a 2001 yz426f that I got from another individual. It didn't run and I knew that from the get-go, so I'm working my way through the whole deal: sprockets, guides, fork seals, bars, levers, controls, etc..etc...

The top end looks very new compared to the rest of the bike. It's very obvious that something has been done to it, perhaps a new head or a rebuild, not really sure. I noticed when I got the bike that the cam timing was off by one tooth. The other thing I noticed is that after a few kicks, it wouldn't get much compression. I retimed the cams as per the manual, but still no luck with starting. Pulled the carb off for a rebuild, which it sorely needed and while I was at it, looked at the valve clearance.

Went through valve clearance routine with feeler gauges, removed the cams and checked the shims. Turns out two of the intake valves were out of spec...one of them by a lot. I started looking at the spring/retainer heights and noticed the one that was really out of spec was a lot further sunken in the head than the others, so based on some previous reading thought there might be crud on the valve and therefore holding it open. Used my small brass hammer and brass punch to tap a little on the out of spec valve and notice that it will actually go all the way up, but it will stick open if you just tap on it a little bit. Hmmmmmm.

I used my thumbs to press on all of the valves. All have good spring pressure, but the three valves that are "in spec" move easily in the bore without any notable drag. The two out of spec valves have noticeable drag and are harder to push down.

Any experience with this? I feel like I might have 2 bent valves, but why would 2 be bent on the intake side and not the 3rd? They all go down at the same time. I think I'm going to be pulling the head.

Thanks,

Dean

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You can remove the carb and look at the exposed valve stems to see what sort of carbon they have built up on them. As to why the outers might bend when the center doesn't, the outers meet the piston at more of an angle than the center does, and a slight collision of the two may be more prone to bend the outers than the center valve.

Either which way, it sounds like you'll need to have a look.

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Yep. Two bent valves with pictures to prove it. The center looks like it took a hit, but didn't get bent because of the angle and the clearance is "just" enough. That being said, I'll replace all three intake valves and leave the exhaust since they look new and are in good shape. I plan on replacing with steel valves and the appropriate '00 model springs. I can keep the retainers and other hardware the same, correct?

How about tensioners? Is there an advantage to going to a manual tensioner vs. the stock spring tensioner from Yamaha? Is that advisable?

Thanks again,

Dean

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For the sake of posterity and future head scratchers....here is a pic of the bent valves. Exhaust on the left and intake on the right. You can see the semi-circular mark where the piston contacted the intake valves. Also note the the valve on the upper right has a noticeable cant and is not resting on the valve seat. Yep.....them're bent.

Dean

bent valves.jpg

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How does your piston look? If the hit was hard enough to bend the valves, I would think you would need to replace the piston as well.

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No to manual tensioners for 98% of everyone who owns one of these. The automatic tensioner is a very reliable and magnificently simple device that works perfectly well as long as the cam chain is kept healthy.

You should have a look at the piston skirt and ring lands/grooves after that impact.

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The piston actually looks good. The marks are miniscule and only one will catch a fingernail at all. The others are shiney but no divots in the least. I may be wrong, but I think it sat up and the valves bent while I or the previous owner was kicking it, not while running. I will try to post a pic.

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