I had an engine fire during startup on a AS 350 B due to a broken fuel injection line. The helicopter, an old workhorse with much more than 10000 hours. Engine was hot, only a 3-5 minutes stop on ground during intensive sling ops. Me in cockpit, never had an indication, no lights, no noise. I goes through the startup procedures normally, but suddenly the ground crew became nervous and jumped and waved me to cut the engine. At first they saw a really (their remembrance) big, big white steam cloud of vaporous fuel and i was wondering why the engine is so tired and rounds up slowly. I closed the fuel valve and i believe some seconds later or in the same moment (i never saw it) the cloud ingnited on the hot engine. Grund crew extinguished the fire immediately within seconds and without problems. Surprisingly there was nearly no damage due to the quick reaction of the ground personel. Since that story i become nervous, feeling unhappy and skidding on my seat if an engine is so tired and slower on startup as normally.