[TWM]

....seems as if Bruce Edwards has pinpointed the problem - should be metering fuel and air through the idle circuit. The CFM rating should not be an issue.

When the engine is idling, take a look down the secondary venturi (careful here). If fuel is slobbering (old tech term) into the secondary venturi, it means that the throttle plate isn't closed - when the throttle plate isn't closed, fuel will continue to be pulled through the main metering circuit after shut off and cause the dieseling that you describe. If the problem is intermittent, the throttle may be sticking slightly open, occassionally, for some reason. If the dieseling always occurs, it is time to go to the basic adjustments. As Bruce Edwards mentioned, make sure that the power valve is OK and then get the float level right. Then check for mis-adjusted idle speed screw and idle mixture screws. ...time to hook up the vacuum gauge. Hope that helps.
TWM