Pickett must be running some big, honkin' electrolytic caps in his Speedster.... 
Two questions and a comment:
What RPM is your engine idling at? If it idles above 800 rpm, try turning the idle down (on both carburetors if you have two) to 800 or slightly lower. Most of the time, that can cure it from "dieseling".
What Octane gasoline are you running? If it's 87, try running your tank down close to the reserve and then put in a couple of gallons of 89 or 93. That might cure it, too.
You could also try installing new spark plugs that are one heat range lower than what is in there now. I know there is no ignition so no spark after the engine is turned off, but a hotter plug may be just hot enough to keep the combustion cycle going for a second or two before the plug tips cool off. This is, admittedly, a longer shot than the first two, so try those first.