The Oxygen Sensor does NOT require any power from an external source (ex: computer, etc.) unless talking about a fuel injected engine that is controlled by a computer. In that case there might be a 3rd wire that comes from the computer that acts only to heat the O2 sensor up when first starting the engine. I don't think you need to worry about this though.
In your case with a carb. I assume your sensor only has 1 wire which is where the signal will come from when you hook the (+) lead to a voltmeter (ground is obtained through the threads, just like a spark plug. If the 02 sensor has 2 wires, then there might be a ground wire along with the signal wire, but still hook up the (+) lead to the wire that is the signal wire and ground to a good ground.
The oxygen sensor works by a chemical reaction due to the amount of oxygen that is around the sensor. In this way, it acts like a small battery that creats it's own voltage. So a normal 1 or 2 wire O2 sensor needs no external power source because it makes it's own power.
0.45 volts should in theory be the reading that you are looking for using a carb., right in the middle between rich and lean (you might want it slightly richer but something close to .45 or .5 volts). You could then drive the truck around and have a helper look at the voltmeter and see if it goes lean or rich during different driving conditions. With a little experimenting you should be able to tune the engine well. Remember with a carb., there is always a fraction of a second that the engine will be lean due to the time it takes for the accelerator pump in the carb. to send fuel through the intake.
If this were a computer controlled engine, the O2 sensor sends a voltage to the computer and the computer then tells the fuel injectors to either squirt more or less fuel into the cylinders. If the sensor is working correctly on this type of engine the reading will be constantly changing (at least a new reading once per second). The computer is able to recieve and send data quickly enough, so the engine will run rich for 1 second and then lean the next second and then back to rich the next second.
Hope this answers your questions. Sounds like a neat way to tune up an engine. Hope it works.
Adam
