hi im George i just bought a 83' rx7 for $600, the car is in good condition and starts (somewhat), i do have some question regarding the engine as i am new, the guy i bought it for had it sittin for a while but it does start after 2-3times , ill just jump to the questions cause i really need to get my daily car running! it is a 12A engine!

1. my car cant stay in idle for very long and have to hold a little pin under the carb so it wont stall, how do i fix it?
2. the car oveheats after 20min or so of driving, the temp is just above the C then rises fast and starts to overheat, any ideas on what it might be?
3. what is the choke for? is it only when the engine is cold or for something else?

thanx for any info you can give me!