My mother in laws jd self propelled mower has a B&S 6.75hp engine. It starts ok, but runs low rpms. when the choke opens it starts running rough then shuts off. I can restart it and keep it running if I hold the choke closed, and can get the rpms up if I manually increase the throttle while holding the choke closed or mostly closed. Everything is supposed to be automatic, choke and throttle. Any thoughts on how to fix this problem?