If anyone is having unexplained loss of power at wide throttle, this may be the reason.
I notice that my trusty 2007 XT 225 was not running well above 3/4 throttle. There was no way to get the bike above 45 mph especially on an incline. The power felt great low in the rpm range, and it would handle tight twisty trails just fine. Once the trail opened up or I got onto the street, it became a real problem. I tried changing the jetting, added a DG pipe, using high quality gas, none of it worked to solve the problem. I was actually thinking about getting a new bike..... I know right.....so the search on this forum began and I came across a thread about the coasting enricher becoming cracked and causing problems. Sure enough the top piece of the enricher was missing.
The carb was sucking in too much air and made the wide open throttle condition useless. Sudco had the part so I ordered it along with a rebuild kit; BST 34-101. While I was collecting parts, I thought it might be the diaphragm assembly wearing out as well, so I ordered a new one from Yamaha. Expensive piece, but it was worth it in the long run because the original was severely weathered. That rubber was fourteen years old.
The issue was in the directions with the rebuild kit. The company supplies a new washer that is designed to keep the needle in place so as not to suck up into the diaphragm spring. The problem is that the directions show that it goes below the clip on the needle. As per my experience, this is wrong. The bike will run with it in that position but every time the throttle opens wide, the bike begins to miss, like it is starving for fuel or something. It wants to cut out. letting off of the throttle allows the needle to fall back into its slot and the bike runs fine again. I have an online manual that says to put the washer above the clip, but the picture is of a different style of washer. The manual is correct, the washer is supposed to go on between the top of the needle, above the clip and the bottom of the spring, which prevents the needle from sucking up out of position. The carb was removed a half dozen times, trying the washer in each position.
Conclusion:
Once everything was correctly seated, the bike runs better than it has in a few years. Not sure how the coasting enricher cover broke but the Sudco rep said that it is common.

Glad you figured it out. Got a pic of the correct installation?

I do not have a picture of the correct installation. The washer ( white plastic) that looks like it fits on the bottom of the spring should go on the top of the needle, above the clip. The other small plastic piece is designed to fit just below the clip. A good way to check for proper installation is to physically turn the entire carb upside down while holding it. If the needle stays put in the slide, and does not disappear into the slide, it is correct. You should be able to take a finger and push on the needle without it moving around. The slide should move up and down with the needle.

The entire coasting enricher assembly is available on eBay This is a REAL Mikuni part, NOT Chinese. Yes you can get entire Chinese carburetors for almost the same price, but my experience with Chinese stuff is that it is total junk.

https://www.ebay.com/itm/Genuine-Mikuni-TTR225-XT225-Carburetor-Air-Cut-Off-Valve-Rebuild-Kit-MK-403/173729785445?fits=Model%3AXT225&epid=214785423&hash=item28731a2665:g:xQIAAOSwwVZfyFJS



If you are talking about the main diaphragm/slide assembly, part number 25 https://www.cheapcycleparts.com/oemparts/a/yam/5004369bf8700209bc78c226/carburetor Yes that's expensive, but I have never seen one fail. I did have to replace one on a Harley Sportster once, but the rubber was fine. The metal slide actually had a crack in it. First time I've seen that.

Yup, the parts I got are genuine Mikuni from Sudco. They are a Mikuni Dealer in my area. They had good knowledge of the XT but I was surprised that the directions were not totally accurate. But in their defense, I will say that the directions are simply an exploded view of the carb with the location pointed out for each part in the repair kit.

As far a Chinese carb parts; I would not dream of buying them unless I had no other alternatives, and I would have to think hard about that as well.
That slide diaphragm was damaged due to the amount of dirt and grit that entered in through the broken coasting enricher cover. I was shocked at how much sand got in there through that open port. It’s a wonder the carb worked at all.