I have read counless post regarding dificulty starting the YZ400f and 426f's.
This is just a review of what helped mine.
I have a 99 YZ400f, and when she is cold, it would take 12+ rounds to get her started, once warmed up, she would fire 1st or 2nd kick. I switched to the iridum plug, and now she fires on about the 3rd or 4th round when dead cold. That one change was a god send to me.
Obviouly, this will not solve carb related issues, but I would suggest the iridium plug in all casses.