I have a PowerBoss 8000 generator with a Briggs 16 hp ohv engine. I try to start it every couple months to keep it running good. The last few time I started it the engine idles up and down. I have turned the adjustment screw on the side of the carb(the one with the spring on it), and it idles better now. What would cause it to idle up and down like that, and also there is a black adjustment knob on the front of the carb, not sure what it is, I have turned it and nothing happens. Does anyone know what this adjuster is for, I have attached a picture. Any help is greatly appreciated.