RX7Club.com - Mazda RX7 Forum

This is something I read about tuning Carbs:

here is how you tell what circuit needs to be richer or leaner. You put an instrument in the exhaust flow that tells you graphically whether the engine is lean or rich. You then put a vacuum gauge on the engine to tell you what circuit is working at the particular time that you are lean or rich.
Simple!
The first instrument is known as an O2 (oxygen) sensor. These sensors are used in electronically fuel injected cars as a signal telling the computer whether to supply more or less fuel. O2 sensors create their own electricity in the presence of heat and oxygen. You simply put the sensor in a hot spot in your exhaust flow, then read the electrical output.
There are a couple of ways to put O2 sensors in your exhaust: The trick way, and the cheap way.
The Trick Way
Buy an air/fuel ratio monitor from Edelbrock or K&N. Summit (1-800-230-3030) sells the Edelbrock system for about $130. This comes with an O2 sensor, weld-on bung, and a display panel with yellow, green, and red LED lights representing lean, good, and rich mixtures.
Racer Wholesale (1-800-886-7223) sells several different air/fuel ratio monitors. One has two channels for dual exhausts, and another has rich/lean and injector duty cycle for you FI guys.
The Cheap Way
Buy a single wire O2 sensor at your local parts house, make your own bung, and read the voltage with a digital voltmeter. The sensor that I bought is a Standard brand, number SG-12. The threads on this sensor are the same as a small-block Chevy gasket-style spark plug, so the bung can be made from one of those spark plug anti-foul adapters. Other O2 sensors use the large diameter threads of 18mm big Ford spark plugs. Just cut and fishmouth the adapter so that the sensor sticks into the exhaust flow. You need to put the sensor as close as possible to the engine so it gets hot and stays hot. Just make sure you route the wire so it doesn't get burned by the hot exhaust pipe. Weld the bung to the pipe, then drill and file the hole to clear the sensor.
Since the purpose of this sensor is just a guide to help you tune your carb, not run a fuel injection computer, if you can't get the sensor really close to the engine, don't worry, because it will still work for your purpose._ All that will happen is that your reading may go away during periods of idling._ On the same subject, don't worry about using a heated sensor, as the expense and complications involved are not worth it for carburetor tuning._ Remember, your eyes are using this data, and if it stops for a while, no harm is done!
Sensor installed in Pinto exhaust.

Sensor and welded bung:

Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. I have found my car to run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary.
Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.