Using a specific magic number with the factory O2 sensor to tune A/F is about as useful as using a ouiji board to adjust timing. The o2 sensor's curve is so steep that its almost like an on-off switch. Start with timing in the middle. Turn alky at least halfway up. Have alky turn on at 12psi. Go WOT in 3rd gear. Ignore O2's. What is the KR saying? Is it making a popping sound even without any KR? If so, add 2% fuel...do the same for either situation. If knock goes away, add 2 degrees timing. Does knock/popping come back? If so, add 2% more fuel. Work your way up to max timing. If you're in a situation where you cant get rid of KR or popping without pulling a bunch of timing out, turn the alky up. I like to stay 2% richer than the point where I start getting KR. Even thats a little risky...4% is better for safety. Wherever your O2's end up, thats your happy number. 820's may be rich for one guy, but lean for another guy. My car LOVES 820's. If I even dip into the 700's, my car becomes alot more prone to knock and sensitive to weather changes. As for your crack, the most common place is where primary tubes 3 and 5 are welded together. That weld ALWAYS cracks, and that exhaust leak will make O2's even more worthless. Tune around KR and engine sound, not o2's.
