I like the idea of the marbles, or perhaps old ballbearings over the shoulder, but that's a bit hardcore.
If you didn't mind a small hole in your muffler (old or scrap one, maybe?) to inject parafin oil, it's what the aerobatic planes, Blue Angeles and skywriters all use. In the old days it was used motor oil, or transmission fluid, but parafin oil is effective, clean to deal with and environmentally acceptable. You can get it at most stores for use in tiki torches or lanterns. Hey, get the citronella type and tell everyone it's your way to combat mosquitoes.
