Some more rules/guidelines for percolating coffee:
1) The "thimble'' or well in the bottom of the percolator must be absolutely clean, not blackened with coffee residue (looks like tar) - use steel wool and elbow grease to clean the inside of the well.
2) The tube must be free of blockage
3) The spring on the tube applies downward pressure on the tube, keeping it tight in the well.
Here are the principles for percolating the water in the well:
The heating element encircles the well, heating up the small amount of water in the well to "boiling", which forces the water up the tube, to the top, where it sprays out, over, and through the coffee grounds (this is the repetitive process of percolating) and also slightly heats up the water in the pot. If the inside of the well is coated with insulating material (tar/residue), the water in the well will not heat up properly and will not be brought up to percolating temperature. The water that sprays out of the tube also gradually heats up the rest of the water in the pot. The water traveling up the tube also heats up the water in the coffeepot, so in one "refinement/improvement" for percolators, you may have a tube that has a ceramic or insulating material over most of the length of the tube, to keep the water traveling up the tube as hot as possible before it sprays over the coffee grounds.
However, the heating element is also heating up the floor of the coffeepot, where the thermostat is in intimate contact with the floor. The cold water in the metal pot is also keeping the floor of the pot from reaching or "matching" the temperature of the thermostat, until the repetitive percolating process heats up the water to match the temperature of the floor. The thermostat is supposed to be in intimate contact with the bottom of the floor of the coffeepot, so that when the floor of the coffeepot reaches the thermostat fixed temperature (should be around 194 degrees F), the thermostat "opens" and stops the percolating process (you can hear the "click" when the thermostat opens). If the thermostat is not in good contact with the floor of the pot, it may get hot enough (just from the surrounding air only) to be tricked into thinking that the floor temperature of 194 has been reached and it will shut down the percolating process. If the thermostat is not defective and is in good contact with the metal floor of the pot, it will accurately read or measure only the floor temperature and will not shut down until the floor temperature matches the thermostat fixed temperature setting of 194F. With a sufficiently effective percolating process, the metal floor temperature should also match the coffee/water temperature.
The bottom (interior) of the coffee pot, where the electricals, heating element, thermostat, etc., are housed (trapped), gets very hot - that is why the bottom (black) area of the coffeepot housing is made of a black thermoset material, which will not melt or soften with high temperatures, but can get brittle and crack with prolonged heat and ageing. Note the extensive use of protective silicone material sleeving, high-temperature wire, high temperature terminals, etc., inside the housing - there is a solid reason for that - the heating element is housed in there, and the air inside the housing gets very hot, in the area of 200 degrees F or more, which is enough to trick a thermostat into thinking that the percolating process is completed. Air is a very poor thermal conductor, but if that is all that a thermostat "sees", then that is what it will measure.
Zenon