You probably have an air leak in the klunk line inside the tank. The only way to find this is to take the tank apart and inspect the lines VERY carefully. Bend them over your finger, roll them around, and stretch them. Small holes will become obvious.

Another way to prime the engine is to put your finger over the exhaust outlet and then crank the engine over by hand. This will pressurize the fuel tank and drive fuel to the carb. You can also use this method to see if fuel is leaking out of any air holes in the lines between the fuel tank and the carb.

Finally, don't discount the possiblity of a piece of crud caught in the needle valve or the spray bar assembly. If you have a remote needle, back flush the needle valve with clean fuel. Most carbs are easy to pull the throttle barrel and inspect the spray bar. It doesn't take much crud to keep an engine from running right. We had one yesterday at the field, and when we took the barrel out, there was a tiny spec, smaller than the head of a sewing pin. When we cleared it out, the engine ran great.

Brad