Grey Wire

I am still a little unclear on the effect of cutting the grey wire. After running the bike in on the road I noticed an immediate increase in top speed of about 10mph, but this was combined with cutting the throttle stop.

From the various threads cutting the grey wire moves the ignition timing from WR to YZ characteristics. But if the Euro models do not have this, then why do they have it for the US. I would assume the grey wire restricts the bike along with the throttle stop to pass the US emissions tests? I would assume this restriction is not due to noise levels, as the UK is just as sensitive on noise by my understanding.

Would the grey wire in place help with slower 'Euro forest riding' where you are lucky to hit 30mph and not a tree!? Does this only affect the higher revs?

several reason for this stuff on the US bikes. Its to comply with noise and tailpipe emissions. If it meets a certain standard, it gets the green sticker status. The 03-04 WR450F got red becasue they were unable to meet tail pipe emission. When they choked it down for sound, emissions went up, emissions were OK when the exhaust was opened up, so they were unable to obtain both, just one or the other. Now in 05, the compression ratio came down for US spec'd bikes, I'm sure this was to go green, while the Euro/Candian/AU/NZ stuff remained at 12.5 to 1. I just imported an 04WR450F from Canada and did lots of checking on the Canadian Yamaha website. The grey wire was just one other factor to keep emissions inline

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