I can't say I've experienced that with my Z1 58mm TB's. I was able to get them to close all the way and even after putting them up to the light there was still a trace amount of light getting by the butterfly. I wonder if there's a sharp edge to the butterfly that's catching in the venturi. Perhaps a slight pass with an emery cloth might help. When you say they get stuck after blipping the throttle, is it that "stuck" that pressing on the gas peddle won't release it? That's the first I've heard of that. Have you touched base with Josh or John @ Z1 about this?