I'm assuming that you have a WT668. My guess is one of two things going on. Either the power needle needs to be adjusted or you have dirt blocking an internal passage. You will have to decide which direction you want to go. Do not use the black diaphragm unless you took out a black one. Best bet may be to take it to a good chainsaw mechanic, who will work on these units. Many do not want to touch them, because they have no way to test them under load. This breakdown might help.
http://wem.walbro.com/distributors/servicemanuals/WTseries.pdf