I used to fly a Tartan twin on a Byron Pitts. The way I solved the problem was to close the hole in the cowl where the air came in to the carb. My tartan had a yellow air scoop which pointed into the air stream. When I went to idle on a down line or from full throttle to idle in level flight, the sudden change would cause air to be rammed into the scoop causing the engine to go lean and die. This may or may not be your problem.

Ken