there should be a air filter in there. it is probly sucking in too much
air in that is why when you engage the choke a little bit it will idle.
just put an air filter in and it will/should run good and idle good. or
it could not be getting enough gas make sure the fuel system is not
contaminated. that could be another cause. or it could be over heating.
check the oil.
Is it blowing any smoke at all?
hope this helps,
14yr old moped mech.