Many things can cause that. Anything from a clogged air filter to leaky fuel injectors. Take a scan tool and select live data. Run the engine up to operating temperature, and plug in the scan tool, the data you're looking for is listed under Long Term Fuel Trim. If it has a negative percentage, it's running too rich (too much fuel, not enough air). Replace the air filter, and put CRF 505 Fuel System Cleaner in the gas tank. 

You can also purchase MAF Sensor cleaner and throttle body cleaner as a kit from Walmart. Use the throttle body cleaner on the throttle and MAF cleaner on the MAF sensor. Unplug the MAF sensor before cleaning it, and wait 30 minutes after you've cleaned it before hooking it back up.