Hi andrewskj
In 1973 I was having trouble starting my trainer .19 engine. All of a sudden I saw the covering on the plane disappearing. I took me a second or more to realize that the plane was on fire, since there was no flame visible. I covered it with a large rag to stop the fire. That plane may not have had a muffler, since in those days mufflers were not always used. I have never again had a fire. My guess is that a small piece of a combustable item, such as a piece if leaf or lint, was drawn into the carburator, caught fire and came out of the exhaust still burning.