If you have replaced the needle valves and seats this is probably the cause, i have always found the replacement parts not quite up to the job, so now when i do this job on any carb i polish the tip of the valve and the seat gets a small going over also, to do this i get a piece of stiff card and put a hole in it using a sewing needle i then put a speck of autosol on the hole and push the float needle into it twisting it back and forth i normally do this for about a minute just to polish it up slightly, i then clean it up using white spirit, the seats i polish slightly using autosol and a wooden cocktail stick for about the same amount of time and clean off again with white spirit this takes of any slight burrs on the parts which can let fuel by, i have been doing this for many years and it always works, i also use in-line filters to ensure no particles from the tank get in the carbs and cause problems.