The issue with using the DVM on the narrowband sensor is twofold:
First, the DVM lag and resolution are so low, it's damn near worthless except to tell if the sensor is outputting at all. These narrowband sensors do NOT put a steady-state voltage, but rather a series of spikes and then those spikes are averaged by the EFI ECM for a .0-1V number. In fact, if the sensor is not toggling fast enough,ECM will throw a code for a lazy sensor.
The DVM digits don't refresh enough to keep up with what the sensor is actually putting out. Don't believe me? Go measure your battery, except, while staring at the DVM, stab the battery terminals and immediately release. My Fluke DVM will show a blip but no actual number, and most DVM's won't even register that.
Two: The narrow band sensor is only accurate if it's accurate. Meaning: You have to be damn near where you need to be already, and most carbs cannot stay close enough to where the sensor needs to be during all operating temps and climates. There's a reason that O2 sensors waited until EFI came out...
You use the Wide Band sensor to get every thing tuned, then use the narrow band to make sure it stays there. I bought the MTXL because it's easy to pull it out and swap it from vehicle to vehicle (I use it for tuning Buells, also). If the tuning is not damn-near perfect, the narrow band sensor is worthless.
