Complex sentence: When the turbos were fitted to the later YFM-1, they were plagued by cranky turbo regulators that backfired continuously then an explosion during a September 1939 test flight, made it apparent that the teething engine troubles would not be solved easily.
Split into simpler sentences: When the turbos were fitted to the later YFM-1, they were plagued by cranky turbo regulators that backfired continuously. An explosion during a September 1939 test flight made it apparent that the teething engine troubles would not be solved easily.