JLG 400S won"t start when it is cold out. 25 this A.M. Warmed up to 45 and finally fired. Ran for 15 minutes and quit. Would not start. Waited for an hour then it fired right up. Seems it will only run at a certain engine temp. It's running on propane. will not run at all on gas. Is there a sensor that tells how rich to make the mix? Not familiar with how a propane fuel system works. Thanks for any help.