Just thought I would contribute to this talk of wasp waist burners , as I have gleamed some ideas from this site.
Amal injectors use a simple copper cone for their taper long venturi, fairly easy to replicate either from sheet or pipe, or turn if you have access to a lathe. A morse taper reamer or pin reamer may even be close enough. If you go onto their site you will find a pdf with basic dimensions and pics. Approx taper of 1: 12. Their 3/4" has a venturi of approx 1/2", the 3/4 simply refers to the size of pipe thread.
Currently working on a couple of all steel, water cooled narbs, powered by Amal injectors. Results so far are very promising, in terms of heat output and economy, no pop back and the ability to turn right down. Still prototyping at the moment, so time will tell.
I had considered making my own version of their injector, as I have the necessary machine tools, but for the money just decided to buy them. It would be possible with a bit ingenuity to make a simpler copy with basic tools and pipe fittings. Their jets are very finely sized, so the 3D printer nozzles may be the way to go as a cheap alternative.
Cheers Billy