The first thing to know about Keihin jets is that they are not flow rated like Mikuni. Keihin jets are size calibrated, so a 100 is 1.00mm and a 120 is 1.2mm, so 20% larger yes? NO. Flow through a 120 is related to the area of the jet as well as other variables which don't change.So the area is 1.20 x 1.20 x 3.1412/4 compared to 1.00 x 1.00 x 3.1412/4 or 144% of the flow of a 100 jet. And that is way too much. I would start with a 110 or 112 and see how that works. You may find that stock jets are OK but always better to start safe (rich) and work down.