Not that I think it's urgent to answer a 6 year old question immediately, but in the interest of keeping good info on our forum I'm going to have to disagree with you about that. Improper valve spring tension won't make an engine vibrate. What it will do is cause valve float if it's too weak, leading to catastrophic destruction of the engine pretty quickly. A too strong spring will cost you torque, which our little glow engines are pretty sensitive to anyway. This is an area to not try to jerry rig something.