oh just for the future i never give advice on marriage. so far I havent seen anything that would indicate that the yz isn't going rich or lean.
What I see happening frequently is the bike running rich throughout, the owner not knowing why jets down on top. you can fill in the rest. it may not be the case here, but I start at the cheap spots until i have more to go on. I would bet that if the owner marks the throttle between 1/4 and 1/2 (most are only guessing on that setting until the mark it off) the owner may be able to see if there is a rich or lean condition at that setting and can go from there. could be the float setting, could be brass related. so far i havent seen that he inventoried while he was in there to see how far off or on stock he is on jetting. That can be a tipoff too.
I am not doubting the part about the valve, but I have had lots of bikes that didn't have exhaust power valves act similarly and even the worn out ones can be made to run clean down there if they can run clean on top in almost every case. I see lots of people who rebuild (which is always nice for power) and clean up another condition a little and think that was the whole problem the whole time. 5 hours later, same thing.
The mikunis have that fuel passage behind that emulsifer/needle jet holder that gets partially blocked as well.