Getting an automatic choke thermal sensor to heat and cool at just the right rate is difficult. The ones used on cars chose rather expensive solutions, but achieved good results. The Briggs one should be inexpensive to make, but offers the development engineer very few control parameters to get it working well. If the sensor makes too good a contact with the exhaust port it may heat too slowly, thus keeping the mixture rich too long after a cold start. If it makes too poor a contact with the port it may heat too quickly, so it will not re-apply the choke if the engine is cold-started then immediately stopped and restarted. Perhaps Briggs concluded that cold restarts are hardly ever needed in normal use, or perhaps the extent of the contact with the port is difficult to keep consistent when making thousands of engines.
