On the intake manifold, below the carb there is a 'flap' that opens when the engine is cold and closes once it warms up. All you will see is a rod with a spring on one end, and a weight on the other. Check to see if it turns. If it does not, spray some penetrating oil on it and tap it gently back and forth over time and it shall free up. The problem is if it does not close once the engine warms up, the carb gets to warm and the gas mixture 'evaporates'.
However I don't think that will suddenly happen.
Good Luck.
Colin
