WR to YZ timing - Valve clearances?

Hey guys,

I've done a quick search but couldn't find what I was after. I just wanted to know if the valve clearances need to be adjusted differently for my WR400F after I've advanced the exhaust cam for YZ timing? Or are they to remain at stock specification? Either way I'll need to adjust them, so any info would be appreciated :smirk:

Cheers, Marc

Hey guys,

I've done a quick search but couldn't find what I was after. I just wanted to know if the valve clearances need to be adjusted differently for my WR400F after I've advanced the exhaust cam for YZ timing? Or are they to remain at stock specification? Either way I'll need to adjust them, so any info would be appreciated :smirk:

Cheers, Marc

If I am not mistaken the wr cam spec is advanced 1 tooth compared to the yz. So you need to retard the cam 1 tooth to get the yz spec......any how if you are not changing the cam only it's timing the valve adjustment will be the same

You are correct. The clearances are unchanged, and the WR cam should be RETARDED one tooth to match up to YZ timing.

http://motoman393.thumpertalk.com/tech/yztime.html

BTW, this can ONLY be done to an engine using non auto decompression cams.

Ah yes, I have indeed retarded it. I mistakenly said advanced, since the cam sprocket from a front on perspective is advanced clockwise. Anyway, thanks guys. Just what I needed to know!

BTW, this can ONLY be done to an engine using non auto decompression cams.

I am checking the valves on my 01' w/auto decompression and there are 14pins between the top punch marks (advanced two teeth) is this right? I though the auto was a nice feature when I bought the bike, I didn’t know I would not be able to set it to YZ spec.

I am checking the valves on my 01' w/auto decompression and there are 14pins between the top punch marks (advanced two teeth) is this right? I though the auto was a nice feature when I bought the bike, I didn’t know I would not be able to set it to YZ spec.

If there are 14 pins between cams, the likelihood is that that cam is an OEM Yamaha YZ450 cam, and you have YZ timing already. That's the way a 450 cam sets up in a 426 head.

Gray:

Is there a particular year of YZ cam that may be better than others for the conversion to auto decomp.

In other words is there any cam chain sprocket differences or any cam lobe profile or separation differences that would make one year YZ cam better than another for use in the 426?

Thanks:thumbsup:

In the beginning, the only option was the OEM '03-'05 YZ450 cam. It works quite well, although the tooth profile of all YZ450 sprockets is slightly different and causes the 426 chain to ride visibly higher on the sprocket than normal. In 8 years of doing this swap, no one that I'm aware of has experienced any trouble because of that, though. Hot Cams came out with a drop in e-cam specifically for the 426, but they had problems at first with the auto decompression mechanisms.

Starting with the '06, each YZ450 model year had the cam timing altered to bias the power curve toward high rpm operation at the expense of low end grunt. The '03-'05 cam is a proven good match to the 426 intake, and the '06 would probably also be fine, but I doubt you'd like the '07 or the '08-'09 cam very much.

However, because Yamaha parts prices have gone up so far, and because Hot Cams and other aftermarket grinders have improved their AD products, I think my choice would be a Hot Cams stage I. The chain will fit better and it can be timed just like the stock cam instead of fooling with the pin counting process needed to fudge the 450 cam into a 426.

Thank you for your post GrayRacer, if indeed it is a YZ450 cam is the valve clearance the same as WR426? Can I retard the timing?

This would explain the smaller jetting although the stock WR can probably doesn’t help the cause.

The valve clearance is usually determined by the valves and the head, so I would use the clearances for a 400. If you keep to the tight side (.25mm) it's very close to the same as the 450.

No, you cannot retard the timing, nor would you want to. YZ timing is already retarded from WR, and you won't benefit from going that much farther with it.

On top of that, when you change the timing of an auto-decomp cam, you also change the timing of the decompression cycle. If you retard the cam one tooth, the left exhaust valve will be held open so long by the AD pin that the engine will not generate enough compression to run. Can't do it.

Thanks

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