I hope it's the right forum x_x

What causes a car to run lean? Generally, we can say more air w/o more fuel = lean (duh), but what modifications would end up in a lean running system?

Off the top of my head, the basic modifications that would cause this are:

intake of any kind, including drop-in air filters (but not as bad)

obvious F/I w/o fuel system upgrades

and screwed up o2 sensors.

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But I'm not sure if these would also cause the engine to run lean... this is where I need help! =D

Headers (long tube, 4-1 in this case)

Exhaust (not a huge one... just an Apex'I World Sport)

Any input? x_x