Why do gas weed wackers loose performance?

Answer 1

quite a few possibilities-

engine problems - carb mixture adjustments not correct or muck in carb, throttle or choke not opening fully, not enough oil (or grass in cooling fins) causing overheating and partial seizure, exhaust ports obstructed with carbon

drive probs - clutch slipping, driveshaft not rotating freely, too much line extending from cutter head

using milk instead of petrol - tends to turn to custard which is a poor lubricant