1. Crankshaft bent. This happens if you strike a solid object. Go to an authorized shop and ask if they straighten cranks. They might not do it. If not, have it replaced. DO NOT ATTEMPT TO STRAIGHTEN A CRANKSHAFT BECAUSE IF YOU DO IT WRONG, IT WILL BREAK.
2. Blade(s) out of balance or broken. Have them sharpened and balanced. If broken, replace ASAP!
3. Flywheel has a broken fin. This only happens if the flywheel has been dropped or hit with a hammer. Flywheel must be replaced.
4. Equipment other than the engine is out of balance or damaged. Have this checked out by an authorized dealer for that equipment.