Hi,

As the others have said it has to be an air leak on the inlet side. Check the needle valve assembly by placing a piece of fuel tubing around the shaft of the needle vave, (you have to remove the needle then slide on the fuel tubing over the spray bar then reinsert the needle and reset). It could also be sucking air around the carby body where it fits into the housing with a possibly damaged O ring or through the screws that locate the carby. It could even be a missing screw from the back plate or an ill fitting back plate. Try changing one thing at a time and use the process of elimination.

Good Luck,

Colin