"Dieseling" problem with a travelall

[PGNLV@domain.elided]

	We need some help, please!!  We have a '70 Travelall with a 392, 
four-barrel Carter carb.  It has a rebuilt engine with approx. 40,000 miles 
on it.  The problem is this, occasionally when you turn her off the engine 
doesn't die immediately.  (It "diesels")  Does anyone know what causes this 
or how I may be able fix it??  Thanks!!

Juniper & Sean Abbott

('73 Scout II 304--Herkamer)
('70 Travelall 392--Mithriel)
('76 Scout Terra--Terra Titanic)