The main jets on the B & C bikes are exactly the same as the main jets that fit in the Mikuni VM series carburetors.
Athough they're "the same," they're not
exactly the same. The calibration of the flow rate of the jets uses a rig with a specific pressure head, and where the flow depends on, among other factors, the ratio of the density to the viscosity of the gasoline used for the calibrations. In turn, both the density and viscosity depend on temperature. As a result, it would have taken very little difference in composition of the fuel used by Mikuni along with temperature of their laboratory (a rather chilly 15 ℃ was specified for Amal's) to account for the 18% difference in flow rate. Note that Mikuni's jets actually hit the numbers they should (e.g. 300 cc/min. for a #300), whereas Amal's aren't
exactly the same.
The 18% difference in flow rate is important for anyone who decides to cross-thread a Mikuni jet into an Amal carburetor, since a significantly larger number Mikuni would need to be cross-threaded to give the same mixture strength.