You let the clutch out while pedaling, does the engine steadily turn? So it makes a constant 'bubb bubb bubb bubb bubb' noise? If it's momentary, or it turns over infrequently your clutch is not adjusted properly so it's releasing the pressure plate to full engagement.
Otherwise it could be bad gas, fouled spark plug, no fuel in the carb or faulty ignition, or bad compression. If it's making a pulsating bubb bubb bubb and there's a lot of resistance you're probably OK for compression.