The spring is heated by an zjautoparts electric heating element inside the cover and/or heat from the exhaust manifold that is siphoned up into the housing through a small metal tube. If the heating coil has burned out or is not receiving voltage, or the heat riser is plugged with rust, loose or missing, the choke will not warm up properly. This will cause the choke to say on all the time, or too long, making the engine run rich and idle too fast.
If the bi-metal choke spring is broken, the choke will never close. A cold engine needs a very rich mixture to start, so if the choke isn't working it will suck too much air. A broken choke will also prevent the engine from idling properly (no fast idle while it is warming up) which can cause it to stall until it reaches normal operating temperature.
If the shaft that opens and closes the choke is dirty, it may cause the choke to stick. The same goes for the choke linkage if it is dirty or damaged.
Even if the choke is defective, a choke repair kit or a new bimetal spring should be all that's necessary to eliminate the starting problem. Replacing the entire carburetor is unnecessary and is the same as replacing the engine because the water pump is bad.
Square Bore and Spread Bore Carburetors,.The terminology refers to the difference in size between the primary barrels located in the front and the secondary ones located in the rear. On a spread bore the secondaries are much larger than the primary. On a square bore carburetor all barrels are the same identical size front and rear. Secondaries do not open until extra oomph is needed. The EGR valve spindle open via mechanical linkage or a vacuum diaphragm.