Make sure you have the spring Duey talks about. NExt, buy an inline fuel filter from NAPA or CarQuest (about $10 with hose and fittings) and install it. Clean out the carb and fuel line, then see what happens. That straner in the sediment bowl will stop large chunks, but it's better as a water separator than as a filter - small rust flakes still go through. The inline filter fixed the problems on my B quite nicely.
Clean out the inlet fitting from the gas tank too. Sometimes large rust flakes will go down into the fitting and restrict flow.
The fact that it runs fine for a while makes me believe that it's a trash problem and not a carb problem. You may also have some worn pins or weights in the governor - but I would say trash first.