The key to managing cold starts on propane engines, is to realize that liquid propane's vapor pressure drops substantially in extreme cold, and it will NOT evaporate rapidly without sufficient input heat.
Flipping the tank to get it to start on vapor is not a viable solution on a small liquid withdrawl tank- there simply isn't much evaporation surface to be had- that's why liquid withdrawl into a converter/evaporator is done in the first place.
The mistake most people make, is they fire up the propane engine, then try to run it hard to force it to warm up, which uses vapor FASTER than the evaporator can convert it from liquid to vapor.
Liquid cooled engines circulate engine coolant through the converter/evaporator. Air cooled engines frequently have the converter/evaporator mounted on or near exhaust or cooling shroud, in the flow of engine heat. Until there's sufficient heat there to accellerate evaporation, the evaporator will NOT do it's job, hence, liquid will flow through the demand regulator, mixer and manifold, and effectively 'flood' the engine by displacing all oxygen. hard on the demand reg and mixer.
Proper method- Open the fuel valve, start the engine, and let it idle. If it seems 'floody', then open the fuel valve, then close it, then start the engine, and once running, 'nurse' it by opening the valve every so often... it will eventually develop enough heat to the evap, to work fine on it's own.
Best way, if the temperature is really, really low- disconnect the bottle, and bring it inside, let it warm up. You could also... if it's a steel tank... slap an 'engine heater' magnet to the side of the tank and run it for a few minutes.
At -40F, propane only yields about 1psi... warm it to -25f, you'll get 8psi. For the demand regulator to meter properly, you need 11" water column, which a cold tank can DEVELOP, but until you have evaporator HEAT, it won't be able to MAINTAIN that pressure. Add heat!