The first question should be: Are you running an idle solenoid? If not, then the throttle is open to far (idle speed too high) and most likely the cause of the dieseling...the factory put them there for this very reason. See Dayclona's post above. Advancing the base ignition timing will allow you to close the throttle blades further at idle and should help with the dieseling...but check the total advance and limit the mechanical advance as necessary.