I have had the same exact symptoms when i swapped a15 to a12 with standard carb. Every time i brake at approach to lights the engine stalls. I have to downshift to 2nd, sometimes 1st to keep the revs up or i blip the throttle or i use left foot to brake while out of gear and right foot on gas. Very annoying and potentially dangerous. I always suspected my brakebooster but now i reckon its the float level in the carb as suggested by jmac