Two things with incomplete perks - Actually, the perking stops before the temperature reaches at least 185 degrees F. This is normally caused by the thermostat:
1) Thermostat is defective (must be replace) or
2) Thermostat (flat disk portion) is not contacting the metal pot properly:
a) Should be fairly high pressure "pressing down" on the disk to make good contact with the coffeepot metal floor (senses the temperature of the metal floor). Note: different coffeepot designs use different methods of applying good/high pressure on the disk portion of the thermostat (Examples: piece of silicon rubber, spring metal, etc.) Note that you must not electrically short out the disk - must be electrically isolated and current can only flow through the thermostat through the proper electrical contacts (blades, brass leads/screws, etc.).
b) Use a little bit of thermal paste (same stuff as for computer chips) on the bottom of the disk where it would contact the coffeepot metal floor.
c) Lightly sand the bottom of the disk to remove oxides and also to make sure that the bottom of the disk is flat and thereby makes maximum surface and thermal contact with the coffeepot metal floor.
Do these three things (a, b, & c) with the existing/"Defective" thermostat and run a brewing cycle before you replace (throw away) the existing/"Defective" thermostat (Most of the time, the thermostat is probably OK - I have "salvaged" most thermostats this way). Bottom Line: Thermostat disk must make good thermal/physical contact with the floor of the coffeepot. Period. With poor contact (pressure too light, inadequate surface area, oxides on surface, etc.), thermostat will be "tricked" into thinking that the proper brewing temperature has been reached and will stop the current flow through the thermostat (thermostat contacts will open at the proper brewing temperature, which is at least 185 degrees F). After perking stops, the water/coffee should be measured to confirm proper temperature. Once the thermostat contacts open, then the "Warming Circuit" will keep the coffee at a slightly lower temperature (maybe 10 degrees cooler) forever, until the coffeepot is unplugged.
Note that a thermostat is "Normally Closed" at room temperature (very low resistance - virtually a dead short) and does not open up until a certain temperature is reached - at least 185 degrees F, but could be as high as 205 degrees F - this is a function built into the thermostat and some Thermostats will have the Temperature Stamped on the disk/thermostat.