http://www.historicmotorcycle.org.au/Adler/carby/jets.txt Everybody uses different methodology to establish their number for their given type of jet.
Mikuni for example, use a few different type of main jets in the various carbs commonly found on motorcycle engines.
Each of their different jets are tested and calibrated in different ways.
So for example, the Mikuni Long Hex, Short Hex, and Round-SlottedHead Jets in #71 size are 3 different Calibrations.
Besides the Physical differences, the Flow Rates are different.
Anyway, by whatever method that's used,
Jet Numbers are representative of a VOLUME of fuel.
CC's or Grams, usually per minute
So #71 Mikuni jet would flow about 71cc's of Gasoline per minute.
Mikuni, Dynajet, and Kehin all have different drill sizes for the same jet number:
drill.......................Dell'Orto#.......................drill.......Adler
inches---Kehein#---DynoJets#-----Mikuni#----mm------Bing#
0.035 ----- 92.5 ------- 92 ---------- 86.3 ----- 0.92
0.036 ----- 95 --------- 94 ---------- 88.1 ----- 0.94
0.037 ----- 97.5-------- 96 ---------- 90.0 ----- 0.96
0.038 ---- 100 --------- 98 ---------- 91.9 ----- 0.98
0.039 ---- 102.5------- 100 ---------- 93.8 ----- 1.00 ---- 110
0.040 ---- 105 -------- 102 ---------- 95.6 ----- 1.02
0.041 ---- 107.5------- 104 ---------- 97.5 ----- 1.04 ---- 115
0.042 ---- 110 -------- 106 ---------- 99.4 ----- 1.06
0.043 ---- 112.5------- 108 --------- 101.3 ----- 1.08
0.044 ---- 115 -------- 110 --------- 103.1 ----- 1.10
0.045 ---- 117.5------- 112 --------- 105.0 ----- 1.12 ---- 120
0.046 ---- 120 -------- 114 --------- 106.9 ----- 1.14
0.047 ---- 122.5------- 116 --------- 108.8 ----- 1.16
0.048 ---- 125 -------- 118 --------- 110.6 ----- 1.18
0.049 ---- 127.5------- 120 --------- 112.5 ----- 1.20
0.050 ---- 130 -------- 122 --------- 114.4 ----- 1.22
0.051 ---- 132.5------- 124 --------- 116.3 ----- 1.24
0.052 ---- 135 -------- 126 --------- 118.1 ----- 1.26
0.053 ---- 137.5------- 128 --------- 120.0 ----- 1.28
0.054 ---- 140 -------- 130 --------- 121.9 ----- 1.30
0.055 ---- 142.5------- 132 --------- 123.8 ----- 1.32
0.056 ---- 145 -------- 134 --------- 125.6 ----- 1.34
0.057 ---- 147.5------- 136 --------- 127.5 ----- 1.36
0.058 ---- 150 -------- 138 --------- 129.4 ----- 1.38
0.059 ---- 152.5------- 140 --------- 131.3 ----- 1.40
0.060 ---- 155 -------- 142 --------- 133.1 ----- 1.42
0.061 ---- 157.5------- 144 --------- 135.0 ----- 1.44
0.062 ---- 160 -------- 146 --------- 136.9 ----- 1.46
0.063 ---- 162.5------- 148 --------- 138.8 ----- 1.48
0.064 ---- 165 -------- 150 --------- 140.6 ----- 1.50
0.065 ---- 167.5------- 152 --------- 142.5 ----- 1.52
0.066 ---- 170 -------- 154 --------- 144.4 ----- 1.54
0.067 ---- 172.5------- 156 --------- 146.3 ----- 1.56
0.068 ---- 175 -------- 158 --------- 148.1 ----- 1.58
0.069 ---- 177.5------- 160 --------- 150.0 ----- 1.60
0.070 ---- 180 -------- 162 --------- 151.9 ----- 1.62
0.071 ---- 182.5------- 164 --------- 153.8 ----- 1.64
0.072 ---- 185 -------- 166 --------- 155.6 ----- 1.66
0.073 ---- 187.5------- 168 --------- 157.5 ----- 1.68
0.074 ---- 190 -------- 170 --------- 159.4 ----- 1.70
0.075 ---- 192.5------- 172 --------- 161.3 ----- 1.72
0.076 ---- 195 -------- 174 --------- 163.1 ----- 1.74
0.077 ---- 197.5------- 176 --------- 165.0 ----- 1.76
0.078 ---- 200 -------- 178 --------- 166.9 ----- 1.78
0.079 ---- 202.5------- 180 --------- 168.8 ----- 1.80
0.080 ---- 205 -------- 182 --------- 170.6 ----- 1.82
0.081 ---- 207.5------- 184 --------- 172.5 ----- 1.84
0.082 ---- 210 -------- 186 --------- 174.4 ----- 1.86
0.083 ---- 212.5------- 188 --------- 176.3 ----- 1.88
0.084 ---- 215 -------- 190 --------- 178.1 ----- 1.90
0.085 ---- 217.5------- 192 --------- 180.0 ----- 1.92
0.086 ---- 220 -------- 194 --------- 181.9 ----- 1.94
0.087 ---- 222.5------- 196 --------- 183.7 ----- 1.96
0.088 ---- 225 -------- 198 --------- 185.6 ----- 1.98
0.089 ---- 227.5------- 200 --------- 187.5 ----- 2.00