My BR550 choke knob won't stay choked. It just winds back to open position. To get it to run I have to hold it choked while I pull the rope. At the same time this happened when I get it started it isn't idling low, just runs like at full thottle once it does start. Trigger is working with the cable to the carb. It all just started now that I really need it for the leaves. Any ideas?