I'm not too big a fan of that movement to the right then back to the center -- I think it either needs a left movement mirrored in to it (I've always found mirroring to improve explosions, but that's just me
), or for it to move only upward.
Try playing with wider movement variations -- for example, slow-rising at first then, as it changes to smoke, have it rise faster. or vice-versa. (I'm not talking about framerate variations, just wanna make that clear)