Quite often, when an engine sets up, the needle valve mentioned above will stick to the seat and not open. If the cover with 4 screws has a hole in the middle, you can sometimes free the needle by pushing the diaphram down with a small punch. If the vent hole is off to the side, remove the cover and push down on the diaphram. They will free the needle. This will solve the problem, IF the diaphrams are not damaged, and the passages are not clogged.