Unlikely to be the belt. Mostl likely: the transaxle is starting to show the first sign of wear out. If the reverse speed seems to get progressively slower the longer you mow, that's almost certainly transaxle wear. L100 series hydrostatic transaxles were a fairly light duty design from Tuff Torq and rapid wearout is a widespread problem. See the thread on "K46 rebuild & repair guide" to learn options for fixing the transaxle. Options are: 1) buy new transaxle and install (cheapest place to purchase is ebay where new K46V or T40A (L118's used both and they're interchangeable) are about $500 and installing it yourself is pretty easy; 2) Hunt down a used T40A or K46V on ebay or craigslist known to have low hours and running fine. Those seem to go for $200-$350 but you never know how much life is left in the thing; 3) open up your failing transaxle and replace ($300) or grind/polish (minimal cost) the parts that are failing...if you go this route, beyond fixing the 3 specific parts causing your slowdown, if you have an older serial# you may want to do some additional upgrades which Tuff Torq made in later serial#'s to correct other breakage problems. Study the K46 Repair/Rebuild thread to learn more...
