so y do we adjust stuff on new atvs?

are all new machines required to be adjusted after the 100 miles brake in period? on my first rancher never did anything and it still runs fine?, although friend picked up a rancher with like 65 miles on it and the engine sounds like it makes a clicking noise sometimes when your rolling and press the throttle lightly? is this adjustment, I think its shitty oil and gas with stabilizer is not helping either? HELP LOL :hail: thanks