I picked up a 05 Kfx80 and have a few questions?

1) quad only runs when valve is in the "prime" position... The "on" position does not let fuel flow

2) when I turn the throttle screw in the my kid to ride, it acts like the clutch is slipping... It's that normal or actually a slipping clutch?

3) any good links to brake jobs... Probably needs front and rear soon

4) what other maintenance should I take care of asap for a 12 year old used quad? Tranny oil?

 

Clean the carb, service the clutchs. Changing the gearbox oil is a good thing.
You can read up on the whole petcock thing.
Go from there.