what is making my ford ranger run lean?

i have a 99 ford ranger 2.3L that i just pulled the motor out and had it rebuilt and then i put it back in and ever since i put it back in it has been running lean. Ive tested for vacuum leaks and changed O2 sensors (both) gave it a tune up, fuel filter, mass air flow sensor (a brand new one) new air filter, what else could it be?
 
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