Choke(or stuck open enrichment valve) could do it. I remember years back when manual chokes on cars were common, we had a lady customer bring us her car for a running issue. It would always run good cold but sputter when it got hot. We could never figure it out until one day my boss suggested she leave the car with us and so I was elected to go home with her and return with the car. As she gets in the car she pulls out the choke and hangs her purse on it. I asked her if she did that all the time. Turns out she did. I left it at that until the car started to sputter and as she said there you see what it does I took her purse off the choke pushed it in and the car to her total amazement ran like a dream - go figure