You may want to install a new Oxygen sensor to start with, a new sensor has better frequency and less hang time when switching from peak to peak. It will respond much faster and once you set the engines fuel mixture you will be surprised with with the effect it has on your the tail pipe readings. The fastest way to set your fuel mixture is by checking the EHA current, yours should be set to approx 0 mA (+/- 5 mA fluctuating). To avoid erroneous meter readings connect your ammeter while the engine and ignition key are off! Once you have the meter connected run the engine up to about 2-K RPMs for approx 2 minutes and then bring it down to idle. Measure the EHAs idle current, adjust fuel mixture if needed. Now measure the EHAs cruising current at 2-K RPMs. Ideally you'll want to stay as close to a 10% differentail as possible, subtle intake leaks can contribute to sizable EHA current differentials (IDLE VS. CRUISE).