Technical What do you guys think of this plug?

Get some vacuum readings next thing to an AFR gauge when tuning.

 

Have you put a vacuum gauge on it?
Maybe the power piston needs a different spring on it.

 

The back fire is caused by raw fuel being pulled into the exhaust system, (what saltflats said, vacuum below throttle plates is sucking fuel) and mixing with the right amount of air and then boom. . There's a freash air leak in there some where. Good luck, could be anywhere.

 

Or the stem is stuck in the up position not opening the power valve or power valve concert is blocked. A lean can kill milage the best milage comes with the least amount of throttle opening a lean mixture will require more throttle for the same power.