There are no gears in your mower .
The "gear" positions simply limit the travel of the tensioning arm, part No 48 in the diagram below
Both belts should always be replaced as a pair and they must be OEM or OEM spec as they are not a standard profile .
The system is a Tug-O-War between the tension in the top belt & the tension in the lower belt.
Whichever one is stronger will cause the sliding sheave in the variable pulley to open away thus making the effective diameter of that pulley to be smaller & the other to be larger .
The tension in the lower belt is provided by the spring , part No 52
So either the belts are wrong or the spring is in the wrong place or the sliding section of the double pulley , part No 41 is stuck . Transmission diagram
