I have an L28E with L-Jet EFI.
I was thinking of building an AFR (air-fuel ratio) gauge using some common electronics stuff. It lights up LEDs from a signal generated by a narrow-band oxygen sensor in the exhaust pipe.
But, I question the legitimacy and accuracy of the gauge. How does oxygen in the exhaust tell you how much air and fuel is going into the cylinders? There is no sensor that will do that, and it would go into the intake manifold, not the exhaust pipe. The best you can do is to use a modern engine, with a programmable controller to measure the air (via the AFM) and calculate the amount of fuel inject from the fuel pressure and the injection timing, then calculate the AFR. Measuring the oxygen in the exhaust does not tell you what is going into the cylinders. The best you can do with that method is to guess at what the AFR is, with presumptions of engine perfection (you would need an engine that is perfectly built, in perfect operating condition, producing a 100% perfect combustion of what did go into the cylinder.)
So using one of the so-called AFR meters is a bunch of hooey, and the value it gives you cannot be trusted. Using a narrow-band oxygen sensor, the signal will jump around, from a relatively lean (say 15:1) to a relatively rich (about 12:1) mixture- IF you believe the theory of these sensors. It's logical to say that the amount of oxygen detected by the sensor, is proportional to an amount f air, but it cannot tell you the AFR. It only tells you there is oxygen/air in the exhaust.
I have read a lot about wide-band sensors, but I'd say it is the same as any other oxygen sensor.
The AFR display I will be building, if you are electronically-skilled, is based on the LM3814 LED driver chip, and lights LEDs, linearly, in proportion to the signal. And the best you can do is to adjust the mixture so the display moves around enough to get an "average." then do some on-road or dynomometer testing to get your desired mixture. Or, you could use an exhaut gas analyzer to adjust the mixture based on theCO content, whic is similar to the oxygen content adjustments.