Performing a full tune-up and replacing the fuel lines on a Troy-Bilt TB525CS 4-cycle string trimmer. I replaced the air filter and spark plug and changed the oil. I also greased the trimmer head gearbox and the flex shaft. I cleaned the carburetor in a separate video ( https://youtu.be/G6noZ99bC4U ), which I highly recommend when replacing rotted fuel lines. This procedure can be applied to perform a tune-up on most string trimmers. See the links below for the parts and tools I used:

Trimmer Fuel Line:
https://amzn.to/3db0zl8 (affiliate*)

Hemostat Locking Forceps:
https://amzn.to/2Uxik7l (affiliate*)

Trimmer Air Filter:
https://amzn.to/2THiZSC (affiliate*)

Oil Drain Pan:
https://amzn.to/3bDAauY (affiliate*)

RDZ4H Spark Plug:
https://www.ebay.com/itm/264680302658

Spark Plug Gap Tool:
https://amzn.to/3bITNlw (affiliate*)

High Temp Grease:
https://amzn.to/36Ae8b7 (affiliate*)

*Purchasing from these Amazon affiliate links earns my channel a small commission. Think of it as a tip that doesn't cost you anything. Thanks for the support!