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Oxygen sensor for Carb adjustment??

3K views 3 replies 2 participants last post by  pontiac58 
G
#1 ·
Some time ago I remember seeing a reference to someone using stock parts to make a air/fuel sensor for adjusting carbs. They used the stock Oxygen sensor. I know there are kits to do this but they cost around $50 and I would hate to get one and find out it is little more then a $5 voltmeter.

If any one has made an Oxygen sensor, what was needed and how was it wired.

The reason I ask is I just found a source for MC2100 jets (http://www.carburetorfactory.com/) and would like to start fine tuning my 258.

bandhmo
 
#2 ·
It sounds like a good idea at first, but it really won't tell you very much. On a carb, they only control 10% of the main mixture, and only at idle and light cruise conditions. The rest of the time, the pcm goes back into open loop. mostly for on demand power and other drivability issues. True, and O2 sensor will tell you rich and lean conditions, but you have to know the centering voltage to know this.

A pcm sends down a centering voltage to the O2 sensor for calibration and self test conditions. This voltage can normally be determined by reading O2 voltage when the car is cold, and in open loop. The O2 sensor must be hot and rich to produce a voltage, somewhere between 0 and .900 volts dc. The centering voltage on older cars with a carb usually run about .450 volts dc. If you know the centering voltage, you can determine if the sensor is rich or lean. Warm, .450 volts will be at 14.7 :1. Above 14.7 : 1 is the norm for most carbed cars most of the time. With 12.0 :1 being max horsepower, around 5.6% CO. (WOT, power valve open) Trying to run your fixed jet carb at these lean mixtures will cause serious drivability issues, with possible engine damage.

About the only real good method of fine tuning the jetting on your carb is by rpm. Assuming you have the main and power valve balanced, you can trim your idle circuits easily. First pull the pcv valve from the valve cover, and let it draw fresh air. This eliminates any fuel dilution in the oil, and gives you a constant. Then turn it idle mixture screw in and out until you get the highest rpm possible. Then take the idle stop screw in, and raise the idle to 2500 rpm. Let it stabilize, and note if the engine revs up or slows down when the idle mixture screw is turned out. If it raises when turned out, your idle fuel correction jet is too small. If rpm drops when turned out, it is too big.

 
G
#3 ·
All you need is the O2 sensor and a decent volt meter.
Use the 1 wire type of O2 sensor which is stock on Jeeps, not the 4 wire type that you will find on some of the imports. With those types you have to apply a voltage to make a heating element inside the sensor work. The stock 1 wire type doesn't use a heater, you just have to install it in the exhaust system reasonably close to the head and then wait a few minutes for it to warm up before reading the voltage. My exhaust manifold already had the hole for it.
The sensor makes its own voltage so you don't have to apply any power to it. It's like a really weak battery so all you have to do is put a decent "DIGITAL" volt meter on it. The meter has to be a good one so that it doesn't load down the voltage that is being produced by the sensor. All meters have a spec call "Input Impedance" which is a measurement of the amount of load the meter puts on the circut while its measuring voltage. Most decent digital meters have an input impedance of greater then 10 million ohms, a cheap digital meter that should still work might have a spec of 1 million ohms, a cheap analog meter which will not work might have a spec of 10,000 ohms or less.
So just get the O2 sensor and hook up a decent meter to it.
Set the meter on a range that will measure 0 to 1 volt DC and once the sensor is heated up it will read above 0.5 volts for anything rich and below 0.5 volts for lean. I think I got that the right way around.
I did this on my 258 with the BBD and I found that if I didn't set it so the idle was running a bit on the rich side it would tend to miss. Perhaps when I do the TR upgrade on the ignition I might be able to lean it back out.


 
#4 ·
A friend put on of those kits on his 258 (cough)street(cough) jeep. Well after his mechanic spent two hours trying to dial in the carb to make my friend happy. The mechanic cut the wires to the damn O2 gauge. Unless you really run the jeep in normal driving conditions you will never get a good reading. Then if you set it to were you think is right. You may end up losing some power. You have a carb adjust it and leave it alone.

Can someone please bring me a hammer my 33's just ate my fender again. Damn i need more lift.
 
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