Carb Parts Warehouse. DO NOT get cheep azz jets off Egay, they may not be accurate. Use only genuine Mikuni jets. Sudco and Jets Are Us are other places.
There's a few places, jets-r-us for example. Only buy individual genuine mukini jets, no jet kits. Listen to larry on the leaktest it is extremely important
Don't hold your breath hoping for a wild powwa increase. Even if you took a stock bore and hogged it out to max of 68mm from 66mm, doubt there would be 1hp increase.
could he be sucking the carb bowl dry? maybe he isn't getting fuel fast enough into the carb. runs it till its dry, wait a few for it to fill again and the process starts all over?