Everything that w8ye and YUKONFLYR said plus

Form personal experience a lot of these engines are supplied with a poor o'ring on the carb mounting. Remove the carb and replace the o'ring that seals the carb to the engine. At the same time I like to put a little high temp silicone on the carb retainer just to make sure that it doesn't leak air either. We have one particular one that was given to us because the owner gave up on it doing exactly what you describe - a simple change of o'ring and it now runs quite sweetly.