Propane needs a regulator, you need to get it down to <1 PSI. then restrict the flow furthur by using a very small orafice...
You can get 12V solnoid valves at a boat shop, (they are mandatory on boats in the US w/ propane stoves and such.
Be careful and don't blow yourself up.
have you read this?
http://www.mrsharkey.com/lpg.htm
its a well written beginners guide to propane fumigation.