After my first comment...before the edit

I went and checked, and I think that the maxair uses mikuni's. The 180 mikuni is overkill in my opinion. However, the size of the
main, is really only an issue when your at
WOT(unless its to small). If you aren't always racing around at WOT, you could have a jet the size of a water hose and it probably wouldn't matter.
I think whats causing you trouble, is the clip in the 3rd groove. Its making the transition between the
pilot circuit, and the main circuit to lean. That's why Curt was suggesting the extra washer under the white plastic spacer. That'll give you more fuel in the midrange(what I called the overlap). The 37.5 pilot, is helping to ease the lean in the midrange, but the clip in the 3rd, is retarding the main circuit.
The reason the bike runs better at WOT, is because the slide has lifted the needle, and the fuel is flowing freer. Your out of the midrange at that point. When your just driving along, and the bike is at 1/4 throttle, the needle isn't letting enough fuel past, because the clip is in the third.
Just my thought on this,...it seems like Maxair realizes that the bike will need more fuel in the pilot and midrange, so they put the 37.5 pilot in to cover that, but then they choke off the overlap by putting the clip in the 3rd groove

. This is causing a lean lag, crappy running in the midrange(cruising in 5th, at 55 to 65 mph).
I have the 37.5 in mine, and am happy with it, and I'm running the poormans
BAK. You really should be fine, because you can use the
PMS to lean it out if needed.
Here is a link I think Curt posted before for mikuni's..
http://www.pjmotorsports.com/mikuni-jets.html
If the kit didn't have an extra washer, you can put the clip in the 4th and see what happens. You may have to turn the PMS in just a bit more if you try that. You can do it with the
carb on the bike, just pull the tank, and the top of the carb is right there, Remove the two screws, take off the cap, and lift out the diapragm and slide, and move the clip.
