Some things to check to avoid going crazy are:
No air leaks... look very close including IN the tank. carb parts junctions, back plate and head. ANY air bubbles in the line is a clue. O-ring on base of carb and neddle valve
tank at proper elevation in relation to the carb
FRESH FUEL...spent half a day cussing a perfectly good engine[:@]
good plug but be aware plugs get blamed for almost ALL of the above a lot.