I have a Exmark Metro 21 that acts like the BBC is slipping. When I get into even moderately thick or high grass, the blade starts slowing and quits cutting, even with a freshly sharpened blade. The manual has nothing about raplacement parts or an adjustment on the clutch, other than the brake cable. Is there a way to adjust the clutch? Or do I just need to buy new clutch parts? Haven't taken it apart yet, but the manual doesn't cover this portion. I'm assuming they would rather you take it to an authorized mechanic. Any ideas? TIA!