check for vacuum leaks around the intake.. if you have power brakes make sure the booster is not leaking vacuum it will cause a lean pop. the next thing is are you running a mech or hyd cam?? to much preload on a lifter will cause it to. just a base line put as much initial timing you can 20deg or more is fine total 38deg have all the timing in before 2500rpms. did you make sure the timing pointer was dead on??