Glad you got it sorted easily. Was going to suggest a hose might have blown off. I have an old 1974 engine driven welder that backfires occasionally when you light off a rod and blows a hose off. Same deal. The great thing about these M103 engines is that they are pretty much 1940s technology from their simplistic standpoint. Even the injection system is pretty stupid simple, except the computer controls for idle and EHA.