Jetting ?

3 replies to this topic
  • tctrailrider

Posted November 19, 2002 - 06:33 AM


In 2000 the WR was a 400 and the YZF was a 426. Both bikes have the same carb.. According to my Clymer manual the stock main jet for the WR is 165 and stock for the YZF is 162. So you add 26cc's and uncork and open up and the main gets smaller? This seems backwords to me, what am I missing?

  • Hick

Posted November 19, 2002 - 08:16 AM


This seems backwords to me, what am I missing?

Can I give two answers? Please?

1) The needle.

The WR comes with a D taper, the YZ with an E. This means that the angle of the taper is steeper on the YZ, it is allowing more fuel in to the motor, more quickly, as you open the throttle. So even though the YZ main is smaller the taper on the needle makes up for a lot of that by being less restrictive. Another way of answering this is that, all else being equal, if you went from a D taper to an E taper you would probably need a smaller main jet.

2) Apples & Oranges

It is kind of silly (and confusing, and pointless) to compare WR jetting to YZ. The WR comes with a throttle stop, a ridiculously tiny exhaust restriction insert, and a sealed airbox. If you plan on removing all this silly foof you should go with YZ jetting since you have substantially altered the way the bike breathes. Yamaha had to deliver a machine that would actually run with the foof I mentioned, hence the weird jetting. In my experience an uncorked/uncovered (exhaust/airbox) WR 426 runs MUCH better w/ YZ jetting.

One additional caveat I'll add is that Yamaha goofed on the needle they put in the first YZ 426, it has a lean spot just off the bottom, the needle that comes in the '01 and '02 is a much better choice.

Hope this helps.

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  • tctrailrider

Posted November 19, 2002 - 09:46 AM


Hick, thanks for the explanation, makes sence to me, sort of. At full throttle with the jet needle totally out the main jet should control 100% of the fuel ( I think) If this is true I would think a stock WR would require less fuel than the YZ and a smaller main. I have done all of the free mods except YZ timing, run the stock pipe with a Vor Tip baffle. I bought the bike used with a 170 main jet, stock is 165. It has allways run pretty good but some bog in the lower to mid range. Before the last ride I put in OBELN needle on the third clip. It ran better but I fouled the plug, had not fouled before this. I now have a 165 and 162 main ready to try if it warms up here before spring. So now the new richer needle and the fouled plug make sence. The carb. tuning info. that I have read gives the impression that the main jet has an effect from 3/4 to full throttle. But it has an impact far below 3/4 throttle. My bike runs good at the bottom, I will play with the main until I get it good at the top and then fill in the middle with the clip position.

  • Hick

Posted November 19, 2002 - 01:49 PM


At full throttle with the jet needle totally out the main jet should control 100% of the fuel ( I think).

I’m not 100% sure about that either, but you can’t get anywhere near WOT on a stock WR anyway. It is hard to say what Yamaha was thinking, and since I haven’t spent too much time riding a 426 w/ a D taper needle I can’t really say if it ran good WOT with the stock main (and throttle stop shortened). The exhaust cam timing did not seem to have any affect on jetting, it did idle rougher with the YZ timing and I may have ended up different on idle mix and speed though…

Anyway, the plug fouling:

I wouldn’t start at the main jet if I was having this problem. Seems like pilot and maybe needle would have a bigger affect. It is true I know that the main jet will affect a fairly wide range of throttle positions, but the affects at lower throttle, say below ¾, are not very pronounced. If you were off by a city block it may matter, but one or two sizes will be hard to spot at anything below ¾ throttle IMO.

I’m out of my league here on the 400 cc issue, but I would say 45 pilot, ELN # 4 and 165 main, provided you are not at higher elevations. If you want to skip to the end you might search for other folks running an “E” taper in their 400 (check the YZ side too). Pay attention to older posts by TTalk member James Dean, he used to have a WR 400 and I know he ran an E needle in it, I don’t think it was an ELN though…

You might want to try another needle, I think an EKP (1/2 clip leaner, one size leaner off pilot on straight diameter) would be a good one to compare with the ELN, try clip # 4. You can get an EKP off an ’00 YZF fiche. Due to a typo all the ’00 fiches (fishes?) say EVP, EVQ etc., ignore that, the “V” is really a “K.”

The bog:

Accelerator pump modifications are all the rage around here, you can affix a small wheel collar, like off an RC car, to the rod that operates the pump, and the collar will act as a pump stop when it hits the carb body. Loosen the collar and move up or down to increase/decrease the pump stroke and amount of fuel delivered by the accelerator pump. I think everybody who has tried this has preferred the modified pump over stock.

Here’s a link to a post on the wheel collar subject:

10p block connector….for Taffy mod???

Hope this helps.


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