The most likely problem is the point where the GAV brass needle valve goes into the brass collar that in turn screws into the angled opening of the carb. The needle valve can be backed all the way out, and you can remove the collar unit from the carb. You will see that on the tip of the collar there is a steel spring clip that supplies tension to the needle valve. First, I would shine up the shaft of the needle valve where it goes into the collar. I would also check the collar for dirt.
Another thing you could check is the rubber bushings on the hangars under the gas tank. Maybe they are sticky or maybe even the GAV rod is bent.