News flash.

I was riding up at Marysville in Victoria over the weekend and noticed that the new airbox is alot more susseptible to altitude than the original. No, I am not imagining it. 2 months ago I rode the WR in dirt mode up here and I had no problems. 180, 48, 1.75 turns out. I rode the WR on rode mode with the new airbox and 180, 48, 2.25 turns out and when I reached about 3000feet it really started to **** itself. Had to turn back and ride lower altitude. When I reached a lower altitude she was fine...

Another aspect to consider is cold air. Zero degrees. Since the airbox has a larger volume, can plenty of cold air impact on the bikes performance?


Carbs do have to be re-jetted for winter temps. They should run lean due to less humidity (and denser air?).

Also I have been thinking about this airbox. I am sure I am missing something, but wouldn't the airbox be at it's best at high engine speeds when the engine is drawing the maximum amount of air? I would think the effect would not be as pronounced at lower engine speeds where air flow into the engine would be less. Yet this airbox is supposed to boost low end response, just like you stated.

Can you help me out with this Mitch, Clark, James Dean?

[This message has been edited by Kevin in New Hampshire (edited 07-03-2000).]

OK, here goes.

At lower speeds and I was speeding up, the bike would cough and hesitate. Once on the pipe it would fly..... and be smooth.

When at lower altitudes, it ran great.

2 weeks ago some mates and I went for a ride in the Brindabella's just out of Canberra, and early in the morning when the air temp was about 3 degrees C, the 3 WR's and KTM400 all ran very poor, about 1 hour later when the air temp warmed to about 8 degrees they were running a lot better. It seems to me that they don't like extreme changes in air temp or altitude ?

Also has anyone tried the cheaper CF air box that uses the standard boot ?


RodH Canberra, Australia<A HREF="" TARGET=_blank>

400 Thumpers Australia</A>

2000 WR400F See Photo's and Modifications

Also has anyone tried the cheaper CF air box that uses the standard boot ?

No. BUT. It could be a possibility for me in the near future! :)

I think the performance gains as far as power are probably 50/50 in the airbox and velocity stack.

The airbox though is lighter, but won't make the bikes engine quicker, but increases dead air volume.

It would be an expensive mistake if just the airbox wasn't what you hoped for as the velocity stack arangement is 1 piece with the carbon airbox.. while the airbox alone uses your stock boot.



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

Discounts for ThumperTalk members.

Hey guys,

If you ride often in the mountains, you will notice cold pockets of air in the valleys. This is because it is denser and sinks instead of rising – hence more oxygen! Early in the mornings the eastern faces will warm up first. Western faces and the valleys will be cold. Of course we don’t have much 02 up here anyway, so those temperature differences make a big difference. We see it right after a rain also. The air is denser and generally the bikes run better.



Thanks guys. Now I understand why the bike runs like kaka in the cold.

Sounds like rejetting is required in the future.


You HAVE TO try the YZ426 carb needle, it will richen the mixture at 1/2 to 3/4 throttle. The cold air and hesitating at lower engine speeds mid throttle are the indicators. The airbox just adds to the argument. The parts listing is EVR, but in fact the actual part will be EKR. I would really encourage you to get the EKP and under the YZ426 part listing as EVP.

Part # 5JG-14916-EP

James Dean

Hey James,

Thanks. I will try that this friday, I am heading down to Winton Raceway and will install the new needle on thursday.

Which clip position would you suggest?


I may have to try it too, as I have the same problem as Mitch when it's cold and getting into the higher altitude.


RodH Canberra, Australia<A HREF="" TARGET=_blank>

400 Thumpers Australia</A>

2000 WR400F See Photo's and Modifications

Probably should start with EKP in clip position #3. EKR is listed as clip #4 for the YZ426 and would be close to the same down low.


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