OK, this is getting complicated. Let me give everybody my recollection of the choke system without having one in hand. The main part of the automatic choke is the bi-metal spring, part of the choke cover, item 56. It is a coiled-up flat spring that winds/unwinds as the temperature changes. Very common on old cars if anybody remembers them. When the motor (and spring) are cold (let’s say room temperature or less) the bi-metal spring is putting pressure on the choke shaft lever, holding the choke closed. As the motor (and choke) warms up, the bi-metal spring unwinds, allowing the choke to open.
Now: The above described system would continue to choke the motor after it is started, causing it to flood out. So, two ways of preventing that are provided. The first is the vacuum pull-off diaphragm. item #55. Mainfold vacuum pulls the diaphragm downward against the spring, which pulls the choke shaft in opposition to the bi-netal spring. The tug-of war is slightly won by the pull-off which allows the choke to open part way. But the bi-metal spring is still trying to close it. This provides a partially closed choke for the warm-up period. As the motor and bi-metal spring warm up, the bi-metal spring unwinds enough to allow complete choke opening. There is a second device that helps open the choke, and actually is what is causing the fluttering. That is the fact that the choke shaft if not centered on the choke butterfly plate See the screw holes on the plate, #91. Since it is off-center, vacuum within the carburetor venturi is sucking on the plate’s off-center side, pulling it open like a swinging door.
Hope this is clear as mud, or better. The system actually is pretty well thought out. Too bad it didn’t work better. It was re-designed in 1964 to operate on hot air with an electric assist solenoid. That worked much better.
