Don't rule out ignition problems but check fuel pressure at carburetor and output volume of fuel pump. Fuel pressure should be 2-4psi. Mine is 2.5psi. Then move on to the carburetor.
When the engine is stopped, look down the throat of the carburetor and manually goose the throttle. Do you see spurts of fuel when you do so? Do this when the engine is hot as it would be when it's causing problems. If you don't see squirts of fuel, your accelerator pump is bad.
If fuel is leaking from the carb into the intake, you may be flooding out. When engine is hot and turned off, Look down the throat with throttle open and see if there is fuel pooled on the intake runner.
What kind of fuel pump are you using? If it's a new mechanical pump, that could be your problem. They are all absolute garbage. Find a 30+ year old USA made original style pump and have it rebuilt.
