First, no spark will definitely show lean on wide-range O2 (Oxygen goes in, and comes back out again --> lean O2)
Pull plug wire, attach plug, ground at threads and look for nice blue spark.
If none, check ignition.
If there, look for air leak. Air leaks downstream of the flapper door means that air is getting into the engine that has not been "accounted for" ---> lean condition. Check that big rubber elbow that comes off the airflow meter. I have had that come off and cause a similar symptom.
