The size 120, is a bore size. It is 1.2mm in bore size. it's THAT SIMPLE. A 135 jet has a 1.35mm bore etc. The 42 pilot is a 0.42mm bore. Remember these are japanes carbs - metric!!
I bought a 1.4mm bit and drilled my 120 main jet to 140. No problem. Yes I could buy some - I'd just have to wait a month! Our dealer has jets for the Harleys (same carbys) and the lowest he could get me was 165.
Drilling is NO PROBLEM - just do it carefully.
