Hi Dr Watts, As Grumpy has said, there are two common causes for this behavior.
The first thing we need to know to narrow it down is what king of engine you are running, It sounds to me like a powertorque, as you have mentioned an O-ring at the top of the crank case.
To be sure, Can you tell me if the starter is at the top of the engine, or on the side? and is the flywheel (fan) on top, or underneath?
