I had a set of weak (mismatched) springs cause valve float on a built sbc, that would backfire through the carb.

 

^ i have always seen it as an erratic drop in power that will get worse and worse if you either keep the rpm going or do pull after pull but in agreement about never seeing a backfire. Maybe misfires but that would be it.
I have seen broken springs that you cannot tell are broken, singles and duals, but the rough idle and issues there should be figured out before anything else happens.

 

That's all I have seen as well, but if the valve pops off the seat during combustion it "could" cause a sneeze or backfire. Be the same essentially as timing being off causing a sneeze or backfire since combustion happens with a valve open.

 

Finally got some hard data in today. We have leakdown done on 5 cylinders and will finish tomorrow. Results: #7 - 6%, #5 - 4%, #3 - 4%, #2 - 13%, and ... #1 - 29%.

Obviously bad news, especially since the bad rocker was on the other bank.