Okay, I think this may be a source for secondary main jets: http://www.motorcyclecarbs.com/Keihi...01-806_C75.cfm
I have one of these Keihin pups on order, and will post whether they really fit the XT350. These come in lots of sizes, and I'm waiting on 112. I know some folks run 118, but that's on a TT (not sure whether the different needle/nozzle is significant). Stock is 106 (yeah, kinda lean)!
Through nobody's fault but my own, I ordered the 112 from Sudco. Nice folks, but I had to buy 4 jets when I only want one. I've been screwing around with other vendors, but either been out of stock or they never got back to me at all. Yeesh. So, I haven't used motorcyclecarbs yet, but they've been good in email.
I'm not sure I need other jets, but I suspect that other Keihin pups will cross to the primary main and pilot.