I only changed the lower belt, and just changed the top one just now after seeing your comment. But the speed didn’t change.Did you replace both belts. Experience has told me they both need to be replaced at the same time.
It seems like the drive idler arm and the variable speed pulley is working normally??Did you check to see that the drive idler arm is moving free keeping constant tension on the drive belt? Second did you check to make sure the variable speed pulley was moving up & down freely when you had both belts off? Both of these could be your problem, along with a bad pulley on the idler arm.
I recently replaced the transmission drive belt, now my mower drives slow going forward, but if I put it in reverse, the speed is normal. Any idea why this is happening? And how do I fix it? Any help would be appreciated! Thank you