is the fuel tank mounted under the engine ? or behind the engine ? if its under the chances are Fuel Draw which means the carby can,t draw and hold fuel in the lines . also is the seal between the carby and engine good ? is the front bearing seal good and not leaking ? the list can go on and on as many issues occur with old or worn engines. start with a richer needles valve setting and also add a pressure fitting from muffler to fuel tank breather. [ 3 turns out on the needles should get the engine running if its only the carby mixtures at fault.