It's a lot easier to set up a mono-fuel system than a dual-fuel system and you need to make sure that you get the best advance curve for each fuel.

Burned valves often indicate excessively rich propane fuel mixtures. Gaseous fuels do not have the evaporative cooling effect that rich mixtures have with gasoline. See Fuels Forum: Combustion Temperature.

Good point about old fuel. Gasoline will become stale if not used continuously while propane never becomes stale. Modern injection conversions always start on gasoline, which helps to keep gasoline in the tank fresh.