You didn't mention WHICH engine you're working on, but almost all dieseling is a function of too-high idle speed. One sneaky trick the manufacturers tried in the mid-70's when they raised idle speeds to lower emissions was to put a solenoid-type throttle stop on the carburetor linkage. "Curb idle" was set with the solenoid turned on, which was attached to ignition switch voltage, and "Slow idle" was set with the regular throttle stop screw, with the solenoid disconnected. They were about 100 RPM different. When the ignition was shut off, the throttle position dropped to the lower speed setting, effectively strangling the engine. You can also shut the engine off with the auto trans in gear, or leave a standard trans vehicle in gear, and let the clutch out as you kill the ignition. Either one will create enough load on the engine to retard the tendency to diesel.
Jerry
