Old fuel smell

I was wondering if anyone could help me with this problem.I have a 1971 plymouth scamp 225/6 1bbl,when driving about 60 mph and let off the gas completely,i notice the smell of old fuel.The engine is tuned good,carb is in good shape,but i cant find the problem.I dont drive the car alot,but it dont sit for very long periods without being started up.The fuel has been drained and replaced,but the smell is still there.Any ideas would help me,Thanks