Check the filter screen if you want but I doubt it's plugged up or very dirty, The symptoms you describe don't indicate that. You are on your third gallon of fuel, has the engine done this from the begining or did it just start this. It sounds like your metering diaphragm lever may be set to high, it should be flush with the top of the surrounding casting, or the pop off pressure is to low, also take the vent hose you put on off, it's very rarely needed on a rear carbed reed inducted engine and I've seen it cause as many problems as it has cured. Do one thing at a time, that way you will know what it is when you find it.