Make sure the belt is tight, if the belt is stretched and loose it may kick back.
Also make sure it has a tilt a cut flywheel on it, same as a mower one except it should have a steel band around it for extra weight, if this is absent and has a mower one It may also kick back.
Lastly it might be worth checking the ignition module, if that has gone faulty over th years the timing could be out.
