[Ben Holt]

O2's will be lean on 2 step (especially if they are just in each collector) because holes are misfiring. Then it will usually show fat when feet are swapped because of the load change on the engine and velocity going into carbs. Some combos are fine this way. Some like to be changed around. If numbers down track are rich, try leaning it down until it slows down. If numbers down track are decent, it's squirters, pumps, bleeds, floats,etc