Harry,
I agree with Kelly about the very small probability of combustion from the start gas. I actually have less discharge using straight propane (I can close the valve prior to releasing the Festo connection to the solenoid), than I do from Powermax, and that is minimal.
Keep your length of tubing from the gas as short as you can (2 inches is great!) and you have minimal gas to worry about. Now add to it that you are 2-3 feet away from the engine, and if you aren't holding the lever down there is minimal gas to vent. How much gas do you smell? same goes for the engine. Be cautious always! but I have not had, nor seen a problem with disconnects...UNLESS the gas valve was left open.
My 2 cents.

Greg