Great prices on your link for linear actuators, puterboy. Their $100 4 inch mini actuator would be perfect for a rack closer.
I use compressed air for my simultaneously operated rack closer and decompression actuators, since my Listeroid is also my air compressor. They are cheap and very powerful, controlled with 12V solenoid valves by my homebrew Picaxe engine controller.
Murphy makes a nice electric rack closer, though not cheap. Electric linear actuators are plentiful on ebay, for a diy rack closer and/or valve lifter. Extending a block under the exhaust tappet lip, ala the manual lifter or SOM would not take much force, and could be solenoid or RC servo driven. My pnematic lifter does in fact lift the valve, unlike the SOM. It really only needs to be used for starting or emergency shut downs if the rack closer fails.
Prior to automating my 6/1, I experimented with emergency shut down methods. One that failed was using a fuel valve solenoid. The suction of the IP made it leak, the engine would keep struggling along at idle. The Murphy brand solenoid valve will hold against the IP suction and will work as a shut down, as reported here by someone else.
For an emergency rack closer, I tried and tested using a brass wire to hold the governor spring end, the brass wire held by two screws of a terminal strip. 12V applied through the brass wire would pop the wire like a fuse, and releasing the spring causes the governor to close the throttle. Works fine but only appropriate for emergency shut downs. A retracting solenoid pin could also release the governor spring with similar effect.
