Why do I keep stalling?

I had a similar issue at one point with my slant. Bought a "new" 1920 off the web and stuck it on there because I thought my old carb was an issue with it starting. The thing would start up and be happy when cold but as soon as it would reach temp would sputter and die. Driving it was difficult, always had to rev or else it would die again. I swapped it out with my original 1920 and boom, problem gone. Chucked the rebuilt one out so unfortunately I'm not sure what caused the issue internally. But like shotgunvic64 said, I would check it out internally to make sure nothing is obviously wrong.