That's a pretty big bumpstick for a metering rod carburetor. Sometimes, the metering rod piston gets dry and tries to hang up in the bore. This is especially true when vacuum drops and they tend to bounce around as with a large camshaft. Have you inspected the metering rod piston and bore closely to make sure they are clean and smooth? Also, use a light oil to lube the piston up in the bore so it will operate more smoothly. I wouldn't use anything heavier than Ford Type F ATF.
