Is this a new carb? I have had many chain saws that "used to run great", that I had to replace the carb diaphrams in. The primer button is there just to get fuel into the pump chamber so it can be drawn through the carb body to the metering side. Every problem like yours that I have had, has been a leaky diaphram. There should only be one or two lines to the tank, but none to the engine. The pump in the carb gets its vacuum impulse through the carb body. Some full size ATV's have a vacuum pulse hose coming from the intake to the seperate fuel pump, but I have never seen this on a smaller engine. I hope I understood the question properly and was able to help.