Your machine is belt drive. As these systems wear, adjustments are needed to keep neutral where it is supposed to be. When the neutral adjustment can no longer be maintained, then parts need to be replaced. I recently serviced an older Gravely walk behind machine that was becoming dangerous to start and operate. I looked over the belt drive system and came to the conclusion that all of it needed to be replaced even up to the hand lever, the parts surprisingly are still available, but the customer balked at the cost.
Check the manual for your machine and make the adjustments or find someone who knows how and will do it for you. If your machine is a high time unit, then you may need to inspect and replace the worn parts of the belt drive system to keep your machine safe to operate.
