CO Adjustment
 

I found a way to make accurate CO adjustment for my 1980 262C, which just had its cylinder head gaskets replaced recently. I had to adjust idle, air/fuel mixture and CO level at the exhaust. The whole fuel injection stuff was in a mess. The smell of the exhaust was so bad that told me that it was totally off and it was nowhere near where it needs to be in order to pass the California SMOG test.

My 1980 262C has a Lambda sensor and a frequency valve to control the emission automatically. Without a shop exhaust tester it is nearly impossible to adjust the emission system correctly. Well, I figured out a way to do it. It seems to have worked. I decided to post it here and see if anyone can tell me if it is correctly done or not.

Basically my adjustment is based on measuring the voltage on the Lambda sensor (narrow band oxygen sensor). If the voltage on the sensor measures around 0.48 volt the CO level is at the lowest level. In reality the voltage will be either near zero (lean) or near 0.9 volt (rich). It is almost impossible that it will stay at 0.48 volt. To adjust the CO level use a 3 mm Allen bit to turn the CO adjustment screw in the AFR. You will find that a slight twist of the screw can cause the sensor voltage to swing from one end to the other. It just won't stay at the middle of 0.48 volt.

The voltage meter needs to have a very high input impedance (greater than 1 mega ohm, 10 mega or higher would be even better). A digital volt meter would be one to have such high input impedance. Analog volt meters do not have high enough input impedance at the 1 volt range. So they can not be used. I used a digital meter to begin with. Very quickly I found it not very useful either. The problem is the digital number on the meter jumps up and down rapidly. It won't tell you much if it keeps on jumping up and down so quickly. You really need an analog meter that gives you an avergae reading, because the needle of the meter can't move that fast so it will give you an average reading. Now that's tough. Where in the world do you have an analog volt meter with a 1 volt range and 10 mega ohm of input impedance?

I found one. It is a VTVM (vacuum tube volt meter). many years ago I picked up from an electronic surplus store a HP 412A VTVM. I dug it out from a pile of stuff in my garage and found it amazingly in working condition still. I then used it to make the CO adjustment.

The adjustment is pretty simple actually. I disconnected the Lambda sensor and hooked up the 412A to measure the voltage on the sensor. The engine is already in operating temperature. I turned the CO adjustment screw to a point where I can see the meter flips from one end to another or 0 volt to 0.9 volt (from lean to rich or the other way around). I have to readjust idle too because the CO adjustment causes the idle to change too. This gives me a base of the CO adjustment screw. If I turn it away from there the meter will swing from rich to lean or lean to rich. At this point I reconnected the Lambda sensor. This is when something interesting happens. The frequency valve now is kicked in trying to bring the CO to the optimal level. The voltage on the sensor starts to swing back and forth centering around 0.48 volt. The idle also revs up and down slightly (almost unnoticeable by listening). The analog meter really makes a big difference here because it clearly shows the average voltage rather than instant and random numbers all over its range. It swings back and forth from about 0.3 volt and 0.65 volt. It swings about once every second. It does not stop at the 0.48 volt.

I can rev up the RPM to above 2000 and still see the meter swing slightly back and forth centering around 0.48 volt. I think this tells me the CO level is still at the lowest when the engine is rev'ed up. I checked the exhaust by smelling it. It was a clean smell. I think the exhaust is clean.

I will try to take the 262C to have a SMOG check as it is now. Can anyone tell me if I am doing it the right way to adjust the CO? I don't want to fail the SMOG check and be forced to spend hundreds of bucks to have a shop doing it for me. I believe the California SMOG check requires my 1980 262C to have a less than 1% of CO.