Here is the older photos updated from original post:


The original flange:



I drilled out the original flange with a 15mm drill bit:







Then I cut away the remaining throat with a metal saw on the drill bit:



I cleaned up the remaining burr with a sanding paper dremel bit:






Now its time for the new 15mm throat.
I bought a piece of 15mm OD and 13mm ID stainless steel pipe.
The reason I chose steel was the tapping would be more durable than aluminum as its only 2mm at each end.



And the 17mm long steel throat gets into the carb flange throat nice and snug.





There is no easy way to bind aluminum to stainless steel,
so I mixed up a small amount of Devcon Steel epoxy (same as JB Weld) and applied to the joining surfaces.
A small amount is more than enough to hold them together and seal for any airleaks.





UPDATE: You can drill a hole thru oppsite side and put a setscrew to secure the throat after a fine application of JB weld.