My troybilt 21 in. Lawn mower. Brigs and stratton eng. 6.75 hp.mod# 126T02-0675-B2. 10072757. Auto choke and auto prime. Starts on first pull every cold start. If I dare let it turn off, such as to empty bag. It will not restart till engine is cool. Maybe 1 hour. New spark plug, new air filter, I cleaned the brass orfice nut on carburetor. Noticed the choke flapper stays open when warm. If I have someone cover carb. Hole or close flapper manually it fires right up. Could the bimetal auto choke be bad? All springs are connected.