1614 idle issue

1977 1614, motor starts and idles great then soon as it warms up idle drops drasticly and it pops a bit through exhuast and acts like it wants to stall. when i give it 3/4 throttle it doesnt sound bad but u can still here a exhaust pop here and there. im not a mechanic by have some experience, im hoping its not a bad valve? what do you guys think.