The added slight restriction of the spark arrester put mine back in the range of the control system..but just. Any less restriction and a chip will be required.
The two brothers exhaust needs a good flowing air filter to go with it and absolutely MUST have a chip. Check your plugs even stock and youll see they are too white. They runs these things with a EPA forced 14.7 AFR to pass emissions. I've got mine running at 12.7 right now and love it. I still have some tuning to do, but it smells good now too ;-)
I just got back from my dealer getting my first service done (literally an hoir ago) and he said the 2 brothers can be ran without a chip..... You say its leaning out? Is it throwing a code? Change the spark plug heat range instead of buying a chip?
Check your plug. They are ghost white. Mine got worse when I put on the Two Brothers and a better flowing air filter.
You gotta think old school... What happens when you add an freer flowing exhaust and air filter to something carbureted? You HAVE to rejet, no exceptions. EFI machines are no different. These machines are set to a specific AFR, and the ECU is basic with few adjustments on its own.
Can you run it with out? Yes you can, but you're wasting your $400 on that exhaust and $40 on the air filter because you wont be able to adjust your map (like rejeting a carb) to unlock the power gains. Not to mention, you risk melt down. Your plugs should have a nice golden brown to them, not a ghost white/grey to them.