Will Grey wire really help

I just talked to my parts guy here in washington and he races his wr 426. He is on his 3rd bike and said that cutting the grey wire does nothing for the 426. He said he even called Yamaha and they said it would have no effect. He did say that on the 250 it was like adding a new bike. Please help with this question. I got to go my wife is hitting on me. :):D:D:D:D

lots of people notice a difference. others dont. But the theory behind it makes sence to me. YOu basically lose some hesitaion on the midrange. Right around the point they check exhaust sound levels so it would pass. That is my interpritation or the whole thing.

I did it and really notice little if any difference in performance.


Cured mid throttle heistation for my 2000wr, but no power increase

It does help as it changes the ignition mapping enough to open up the Bottom-en and Mid-Range that was restricted by the factory to pass the Federal emission control requirement. Additionally on the WR, Removing the ACV and doing the BK Mod will bring out the maximum power in the beast.

Good Luck

Bonzai :)

Hey yamakaze, on a 2002 wr426 do you need to remove the acv?

What is the ACV??

I've done the grey and blue wire on my 98'. The blue didn't do anything but the grey wire made a huge difference on my bike, way more power in the bottom and mid.

The ACV or (Air Cut Valve) is located on the left side (Shifter Side) of the carb on WR model's. The purpose of the ACV is to regulate the injection of air into the mixture when cold starting the bike. It also has a minor application during idle operations. The problem is that Yamaha starting with the 98 model WR jetted these things way too rich (65 SJ/90-100 SAJ). Removing the ACV does several things. First the carb breathes better. there are two vaccume lines on the carb that must be sealed with vaccume plugs from the auto parts store. The only real adjustment that needs to be made once the ACV is removed is to the fuel screw (opened a little). When the ACV is gone you can then manually jet the bike for maximum power.

I don't have my data on the 2002 available. If you look at the carb and their is a round vaccume module mounted to the side with three vac lines attached...two to the carb and one to the intake manifold then the answer is yes you have one. If the side of the carb is clean and you can read the Kehin name on the carb you don't.

Bonzai :)

hey thanks Bill. I will have to try that one. Never heard of that one. i have the 98. Tons of hoses everywhere. Sometimes brush puls them off. I hate that.

Keep in mind that somewhere along the line (like on my '01WR250F), the vacuum lines and air circuit from the intake were integrated into the carb body (i.e., no hoses). The function is still the same, but disabling involves blocking off ports that are now completely inside the ACV cover. Do a search on ACV as there is some discussion on this around last fall I think (check the 250F board as well).

I'm not BK, but I cut grey wire and YZ timed at the same time on my '00WR400. Definitely changed the mid range hit, but I don't know how much each mod contributed. I have all free mods, and a FMF Power Core 4.

I have none of the fancy jetting mods save richening mj, stock needle one notch richer, and one richer on pj. That's it. Runs great, maybe not as strong as some with custom needle and BK mod, but doesn't foul plugs, and virtually no cough. I can ride from 2000 ft to 7000+ and it's fine. I am on the second plug after about 3000 miles, and no complaints. Am I lucky, or are we over-thinking the mods?


“Am I lucky, or are we over-thinking the mods?”

Maybe yes and no. Until some insider from YAMAHA provides some answers…

There’s got to be a way to check what happens. Has anyone tried this: Set the bike up on a dyno, wire in toggle switch on the grey wire and note the difference with a timing light as the switch is toggled?

Or, what about just get the YZ ignition module? :)

Does cutting the gray wire on a WR cause noticeable pre-ignition (pinging) that was never noticed before?


Hello BK!

Brian are you there?

Could I direct this question to you?: What effect does disconnecting the gray wire do? :)

I’m curious on your opinion of disconnecting it.


Canadian and Euro bikes don't come with the Grey wire at all so I think it has something to do with U.S. emissions limits. My .02.

Ultra HIGH compression, low grade, pump gas. It’s amazing these engines run as long as they do. I don’t mind mixing octane booster/fuel conditioner if it needs it. How would know you if you needed it. If the ignition map was altered (remove the grey wire) what would that actually change? Hotter spark? Advanced timing? Cut out to no cut out? :)

There has been speculation or information on TTalk about a year ago that cutting the grey wire maps the ignition same as YZ. Possible advance in mid range and raise rev limit. It seems I rarely hit rev limit any more, so that may be correct. I don't know about mid range, as I mentioned. I altered cam timing to YZ spec at the same time, and it woke up the WR powerband significantly.

I am certain that Taffy's jetting tips are the ticket for pure performance. Many people here on TT have done these and all seem to like the effect. My question is this: do the bikes then become more altitude sensitive as a result? Does it require re-jetting every few thousand feet? If I ride one week at 2000 ft and then next week at 7000, will I likely need to rejet?

What do you think, TAFF?


I think that if you get the jetting right, the bike will run great for most situations in your area. If you routinely travel to other parts of the country you will have to compensate for that. I run between 3500 - 11000 ft and most riding is done in the mid range of the two extremes and I only notice differences( that the fuel screw will fix ) at the extremes. I have the stock paj, maj,needle(Canadian)in the #2 position,38 pj,160 mj , air cut valve flipped around , blue wire removed, air box lid gone ,Uni filter, teed breather and vent hoses and my own stock Euro muffler mod and this set up runs great in my area.

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