Briggs and Stratton running too rich. How can I fix?

I seen where an answer was given to fixing a Briggs and Stratton engine that was running too rich and the answer was too change the diaphram. Where is the diaphram and how do I go about changing it?
It is a Briggs and Stratton 3.5 Classic Displavement 148cc Model 9v TBS158U1 Gira:EM
I can only start the motor by putting some gas in the carberator. Then once it starts it cycles fast and slow and goes through a tank of gas in 15 minutes. The exhaust is blacker than it should be.
I tried adjusting the small springs and cleaning the gunk.