Carb problem.

I have a TV... (motor with plastic carb). it wont stay running. prime and it starts. I took off the carb and cleaned everything out so that is is spotless. This is the second one that i came across that acted this way. The first one I tried rebuilding with same results so i dont want to buy and get the same result with this one.
Am i missing something?
TIA

Carb problem.