[undy]

It sounds like not enough timing on tip-in. Your max timing looks a litlle shy too (34 to 38 typical). I'd bump overall timing about 3 to 4 degrees and see what affect it has on the backfire. You can always move the timing back. Not being familiar with the TWM that much, it could be some mis-set "open loop" programming. That could also explain the cold engine only symptom. Keep us updated on what you find.

Dave