Only way to really be sure what main jet to use is by doing a plug chop(s). That process is detailed in old forums and is truly the only way to jet properly. That said, every bike is different and I was suggesting that it might be safer for you to start richer and work down than the other way around. I am still running a 155 main with almost a stock set up. How and what do you ride? That certainly has some influence. I am old and slow and would be considered a trail rider now. Yes, I get a little spooze sometimes, but I am sure I am not too lean! I think (one man's opinion) you will be just fine with the 42 pilot and the air screw about 1.5 turns out. There are many wonderful tuners on this forum. Read some of the past discussions on jetting. Keep working on it; most agree that jetting is one of the most rewarding and inexpensive things you can do for your motorcycle! Cheers John