The main jet and or Emulsion tube is plugged and they need to be cleaned. This can be accomplished by removing the float bowl. In the same hole that bowl bolt was in you find the main jet. Unscrew it and it will usually fall out. The main jet holds the emulsion tube in place it might fall out or you may have to persuade it come out by going down the throat of the carb. You will see as small brasss tube sticking up from the bootom section of the throat. After the main jet has been removed push it down with a screw driver and they will usually popou the bottom. You will see it has many holes of various sizes drilled around it's circumfrence, clean them all out and reassemble. you should be good to go.