The engine with the bolt on weights have came apart( weights coming off) so I should not have to worry about that on mine. You could balance any crank by drilling it but I'm not worried about it, mine seems to vibrate at idle and runs good when under power. I'm happy with it so far. These engines are in my opinion, the bottom of the barrel rejects, but they are fun and cheap.
Just right for a motor assisted bicycle not meant to be a moped or motorcycle.
Norman

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norms-iron-works.tripod.com
motor assisted bicycle engine headlights that work off of your white power wire. check it out