What specific engine do you have? it might help us to know which engine you had. Also does it have the carb that came with the engine, or did you change it out for a different carb?
What type of muffler are you using? A highly restrictive muffler on a inverted engine with a fuel tank too high in relation to the carb will cause these symptoms too.
Are you using a inline fueling valve? If the fuel line routing is complicated, try simplyfing it to see what happens. I see guys having problems all the time who try to use extra gizmos and gadgets to allow for easier fueling and defueling of a model airplane.