I have a 51 Oliver 77 RC. I call it the reliable one. I was discing today pulling a 7 foot manual Oliver discs. After a round or 2 when the tractor is hot it starts to surge and then stalls. It does it a couple of times and it won't start unless the choke is fully pulled out. At this point it won't pull itself at low or high throttle. When I did get it started I left the choke out about half throttle and I was able to finish and put the discs away.

Can anyone tell me what the trouble might be??  Carb?? 

Thanks