As long as you have fuel in the carb bowl (and you do, if it starts choked), then the engine should start and run for at least 15 seconds without the pump pumping any fuel. It's not like fuel injection. The pump only tops of the carb bowl when the float drops enough to let more in. Trust me, it's not your fuel pump. If it was it would cut out under hard acceleration. It's also not your main jet, because when you are starting hot you don't need to even hit the accelerator normally and you are using the idle circuit to start it. You may have a plugged air bleed though. There are several air bleeds in a model B.
If it is really that hard to start warm, and your idle mixture is correct when the engine is warm, if you shut it off and immediately crank it back up, it should start instantly with no throttle applied unless you have bad compression or a really slow cranking speed. Bad compression is caused by a lot of things, but if it occurs when it is hot only it is usually too tight valves or a bad head gasket. Check the compression, it's really easy. A partial check will tell you what you need to know.
If it is only hard to start after you shut it off and let it sit for a few minutes then it could well be a carb problem. Did you use the thick spacer gasket between the carb and manifold? Did you put the check balls in the power valve circuit in correctly? Did you route the fuel line away from the manifold far enough? Is the manifold heat valve stuck?
