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Air Filter
The scooter is from 2008 and has ~3500 miles on it. I looked at the air filter last week and it seemed white and clean. Is a visual inspection enough to give it a thumbs up? According to the previous owner (a retired gentleman who didn't seem as though he would lie about this), the scooter hasn't been ridden through dirt / dust and was only really used for grocery trips.
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As the other guys have provided good information about the other possible issues, i will concentrate on the filter.
You say white, and this throws me off, as the filter on the agility 50 is orange. Can you see, if the filter is like papery fibers, or like a sponge?
If it is made of paper fibers, dont get it wet, but vacuum clean it or blow high pressure air through it, but if it is sponge, then blow air though it (as your vacuum cleaner might not like the oil), and then pour a little oil on the filter. Then you should press the sponge-filter, so only a little oil is left there.
I dont know about the diffrences of the agility 125 and the agility 50, but in my service book, it sais that the air filter should be cleaned every 1000 km and replaced every 2000 km, even though this is not necersary most of the time.