Rcimp,

If you loosened them up then they are loose and wouldn't get tight by themselves. I don't think 'too loose' would cause anything other than poor performance.

Could you have knocked any plug wires off or done anything else while doing the valves. Could a pushrod have slipped off a lifter. If everything was fine before running the valves then it seems the problem would be something associated with that chore.

Improper valve lash shouldn't make the engine run warmer. Excess advance timing can result in higher operating temperatures and could account for backfiring. A fuel mist coating the carburetor sure sounds like tight valves though.

Stuart