With the 42 Mik a good starting point might be either a 17.5 or a 20 low jet, the 98 needle with the clip in the middle groove (or you can try the stock 97 needle with the clip in the top groove), a 50 pump nozzle and a 160 main.
You can also trim the accel pump, there is a adjusting screw for small flat blade screwdriver near the top of the carb where the throttle cables are and it's best to turn it in until it is flush with the carb body. There is another screw near the accel pump cam that adjusts the beginning of the stroke, you can play a little with this one also, but it doesn't have that much effect and it's easy to end up with a stumble if it comes on too late.
They come with different stuff from the factory depending how and where you bought it, generally a 25 pilot, 160 main, 97 needle and 70 pump nozzle.
Mikuni has exceptionally good tech support and if you call them you might end up talking to Joe Minton. I think they have something called a mileage kit that should be very close to what I use.