Could be that the slide diaphragms (rubber part) have holes in them or maybe they are not sealing well at the top of the carb caps.
I've seen that happen when the grooves in top of the carb body, that the rubber part of the diaphragm mates to, are not clean enough to provide a good seal.
I've seen that happen when the grooves in top of the carb body, that the rubber part of the diaphragm mates to, are not clean enough to provide a good seal.