Could be that the GAV is not sealing well. I think there are tests in Moller's book on checking for leaks.
If you have a replaceable seat, try a new one. If not, try honing it:
You can make a hone out of a couple of GAV's:
http://model-a.org/gav_honing_tool.html
That worked for me.
Also, dumb thing to check - make sure that the compensator jet is really a compensator jet and not a GAV seat. My dad mistakenly put one in and the car ran REALLY rich.
Steve