If I evolve my Stage 1 pokemon while Body Building Dumbbells is still attached to it, does it lose Body Building Dumbbells effects?
Yes. Bodybuilding Dumbbells says 'The Stage 1 Pokémon this card is attached to gets +40 HP'. The bonus is only applied while the pokemon is Stage 1. If it stops being one (through evolving), then the bonus will cease to apply. If it somehow devolves back to Stage 1, the bonus is reapplied.