40 is the size of the butterfly in mm ,so 36 idf have 36mm butterflys.4mm ant going to make a lot of differance unless you have ported/bigvalve heads or big cam.the venturi size can be changed as they slip in so you can have a 36 set up to flow more air than a 40 if it has smaller venturi.is this a new setup or has it run before?could be carb flooding ,to much fuel pressure will cause this as will incorrect float level,idfs are sensitive to this.have you set carb linkage up correctly ,could be its only pulling on on carb on idle.disconect one carb from link ,unscrew idle stops and set them both the same then check idle mixture on each cly (fastest idle then turn in 1/4 turn)basic set up procedior.