First thing first, just tap the bowls with a screw driver handle.
If that doesn't help you need to pull carbs and set float height.
Look up how to do that. Will be in Clymers if you own one. If you don't, buy one.
If that's not it, you need to check the seat and needle. A q tip with some compound, toothpaste, or some cleaner will clean the seat.
But you really need to inspect the rubber tip on the needle. Usually the get grooved and don't seal.
You'll need to order a carb rebuild kit and replace them.