Is air filter new? If not get new, it is cheap and old can be really old, if that is bad it won't get enough air and that could cause all kind of trouble.
I don't think that you can change ignition timing at all.
Glogged injectors, get them cleaned at Bosch shop, costs here around 80 euros, maybe around same there and after that injectors are like new in many cases, more torque is on of good things.
It could be also MAF problem, or if some air can go past MAF so it would make mixture more lean. Aux air valve can be also leaking, I think that let air go past MAF, who can verify this?
Fuel pressure regulator at end of fuel rail, if it does not keep enough pressure then there is less fuel in again.
Sure fuel filters could be cause, fuel pump gets often noisy if filters are really old.
Quite many things actually, but checking and eliminating one by one should reveal real reason behind this possible lean running.
