When I started to tune my bike carburettors I drilled out the original main jets - then realised that actually I should have made them smaller! Unfortunately, it is impossible to drill holes smaller, so I had to either buy new ones, or make my own.
I had a good look at the original jets and realised that they wouldn't be too hard to make on the lathe, although a few fixtures and fittings would be required to achieve a consistent set of jets. This consistency was required to ensure that each one behaved identically to the others when installed in the four carburettors.
These require reasonable competency with a lathe, so I won't describe the technique here - it's a challenge for a beginner, and a piece of cake for an expert! I will offer a diagram showing the most significant measurements.
Currently being drawn up - coming soon!