Sportster engines need big props. But a small prop won’t cause it to flame out and quit after a few seconds. That’s a fuel system problem most likely, or a bad glow plug. Use plenty of oil in your fuel in a Sportster - specially if there is a lot of radial play in the crankshaft bushing.

My recommended props for Sportsters; 10x6 for .20/.28, 12x6 for .45, and 13x6 for .65. I use 23% castor oil in my fuel for bushed crankshaft engines. This gives me the best results albeit with a bit of oil spray from the bushing (not a bad thing other than being a little tedious to clean up).