Similar Problem In The Past!
Take your O2 sensor out of the muffler and clean the carbon off the sensor tip. This can be done with a propane torch. Be careful while doing this, just get the sensor tip to glow red hot long enough to bun off the carbon deposits. Allow it to cool off and reinstall the sensor.
The ECM will now be able to get the necessary input from the O2 sensor and adjust accordingly after allowing the motor to warm up for awhile. Good Luck.