I recently had a John Deere L120 given to my by my uncle. He claims to have replaced the belts recently and is complaining that the unit will not go up slight grade hills. When I mow at my house it will run perfectly fine for aprox 15-20 minutes and then will slow lose its power on hills. When you push the pedal the mower just will not go. There doesn't appear to be any slipping on the belts and the mower will not die it simply will not go. The engine doesn't sound any different then when its mowing. Does this sound familer to anyone and can anyone help out. Thanks

Hi,
I would bet that if you check the serial number, tha tit is one of the first couple of years made which had a less than durable transaxle. Failure was fairly common and the most used fix was a replacement transaxle which is a bit pricey though fairly straight forward.
Other fixes were to replace the pump and motor in the K-46 TuffTorq(Kanzaki) transaxle with the newer versions that have slightly larger pistons in each. This is covered on YouTube in a couple of videos which are:
Part 1 - K46 hydrostatic drive Removal - YouTube
And:
Part 2 - K46 Rebuild - YouTube
With part #AM881473 pump and #AM880485 motor if you are that handy with tools.
Another set of videos dealt with the replacement of the whole transaxle with the one out of the larger G series machine. It is fairly good and annotated, and is at:
How to Upgrade John Deere L-Series to K66 transmission - Part 1 - YouTube
And:
How to Upgrade John Deere L-Series to K66 transmission - Part 2 - YouTube
And involves putting in a K-66 in where the K-46 was. Transaxle is part #AM131257.
Peace,
Clarke
PS The serial number is under the left rear fender and is 13 characters long. Tuff Torq Customer Service is 1(866)572-3441 7:00AM to 3:00PM Eastern Time.