It is the Mixture Full Rich command. My guess is the default plane has mixture set to off and since no mixture exist for the jet but FSX expects it to be full, it simply will not start as it has no fuel. If you have mixture set to one of your quad levers, make sure it is full before trying to start. If you have FSUIPC you can create a custom profile and remove the mixture, assignment that will sometimes solve the problem.

Dave