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MrN2OBelvedere

New to me yz400f

7 posts in this topic

Hey guys I recently picked up an older (98/99) yz400f.

The last dirtbike I had was more than 10 years ago and it was an old bike then...an 87 cr125, so just about anything feels new to me. The bike is very quick by my standards and has all the cool stuff I drooled over years ago, so I'll be happy for awhile with it I'm sure.

I have been doing some research on the bike, and the early evolution of the 4 stroke yzf lineup, and it seems like a pretty impressive piece of machinery.

A little background for myself...my background is in cars, and I am a mechanical engineer, so with this combination of traits I am naturally inclined to mess with my toys. I'm certainly no stranger to the 4-stroke engine, in fact they are much more familiar to me than 2 strokes.

In my initial research on this bike I have found that in 2001 the yz426 switched over to titanium valves. Now the bike that I have is plenty strong, and runs and starts quite well (I do think a good carb cleaning/rebuild is in order though). I have noticed that after sitting at idle for more than a couple of minutes that it starts to puke coolant. I also have read that this is a common issue, so I will go through all of the normal checks to make sure everything is up to snuff. I was thinking that in the worst case that the head gasket could be bad or the head could be cracked.

So this brings me to my first question about this bike. In the event that I had to pull the head, it seems like a worthwhile upgrade could be switching to that 426 head with the titanium valves. Do you guys know if the 426 head will bolt on to the 400 cylinder?

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The valves for the '01 are around $100 each. The stainless valves for yours are closer to $115 for the whole set of 5. There is no measurable increase in power or performance as a direct result. So, what do you think?

The 426 head will bolt to a 400, but it will use either the stainless or Ti valves.

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Titanium valves are way lighter, reducing the mass of the valve train, and technically allow for lighter spring rates. Assuming that Yamaha employed lighter springs, then the engine shoulld rev more quickly and be able to rev somewhat higher with the Ti valves. For me I think it would be a worthwhile upgrade. For sure in the hotrod community, Ti valves are the cats meow!

Is there any reason to think that the valves would need to be replaced? Obviously I wouldn't want to spend $500 on a new set of valves in addition to the 3-400 on the initial cost of the head.

Did Yamaha make any improvements in port geometry when going to the 426 head?

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The rev limiter will still shut it down at the same RPMs no matter what valves you have. If you put Ti valves in you wont be able to tell a difference, I didnt when I went from Ti to stainless in my 426

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Titanium valves are way lighter, reducing the mass of the valve train, and technically allow for lighter spring rates. Assuming that Yamaha employed lighter springs, then the engine shoulld rev more quickly and be able to rev somewhat higher with the Ti valves. For me I think it would be a worthwhile upgrade.

Uh huh. From a strictly technical stand point, you are correct. Ti valves do weigh about half what a steel valve does, and that does allow for better valve control with lighter springs, which Yamaha does in fact use, and it will reduce the parasitic loss within the engine to an extent.

But what I said was, there is no measurable performance gain from switching, and I stand by that. You change, you won't be able to tell one way or the other.

But they are 5 times as expensive.

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yep, I got ss in my '01 YZ426 and no diffrence what so ever. btw, welcome to TT Mr.B:ride:

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that bike is not meant to sit idle that long. start it up and ride it for a while , if in doubt route the radiator's overflow pipe some where you can see it to see if it still pukes fluid(while riding). it is supposed not to.

keep a nice maintenance, if if you're not a top rider keep it simple, use it wisely and you'll have a bike for life

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