Some of the Ryobi 31cc engines use the flywheel to aid in balancing the engine, some do not. (Don't ask me how I know...) One way to find out would be to check the flywheel on a balancer. If it is really WAY out of whack, then the flywheel is helping the crank counterweight to balance the engine. I believe it was only the old engines that used this method.

AV8TOR