If stabilizer wasn't used in the gas to store it, or the carb run dry, the gas in the float bowl may have decomposed and clogged the jets, and small passages.
I would suggest dropping the float bowl and spraying carb cleaner into every orfice you can get to, especially the jets, and take a small wire (like a bristle from a wire brush), and pass it through each jet. Also replace the gas in the tank with fresh. If you see a bad situation in the carb, and spraying doesn't work, you'll have to take the carb off, and soak it in cleaner.
If all is well in the carb, and problem persists, you may have a bad diaphram.