I worked on 2 neglected cheapo folding bikes off
eBay for my niece and her boyfriend.
One tube wouldn't hold air. It turned out that it was so badly split that my attempts at a repair didn't hold so I replaced it with one of the 2 spares that I had bought.
The bikes had been left outdoors so there was a fair amount of rust. The chain on one looked completely wrecked but to my amazement there was only one stiff link. I lubed both chains, left the oil to soak in, and later wiped them down to mop up the surplus. I managed to free the stiff link and the chain moved freely.
I adjusted all 4 v-brakes. They are pretty good now.
I checked the gears, which once again surprised me by working pretty well.
The cups in all the hubs are pitted so I made do with just cleaning and regreasing the hubs. The wheels are true and spin okay, but they rumble.
A few other bits and pieces too...
The owners understand that the bikes are pretty low quality but they will do for shortish slow rides on the local bike paths. If they get the taste for cycling I said that I would be happy to advise them if/when they choose to upgrade!