imported post
you could, depending on when it was last carried out,
change the oil and filter (try 20/50w oil see if that quiets the engine)
change the fuel filter and have the one in the tank(fuel tap) checked and cleaned (partial blockage may cause lumpy running)
Get the crankcase breather hose's and receptors cleaned
check the vaccum hoses are in good condition
clean/change the air filter
check the induction rubber hose's to the carbs for cracking/splits (air entering here will give a weak mixture)
another simple one that has just come to mind, have the fuel tank filler cap checked, make sure the small vent in the capisn't blocked