Did it happen cruising around town? or on a wide open pass?

If it happen just cruising around town. I would say you might have some fuel in your oil that did back fire. With no mech. fuel pump, is your choke set too rich? also do you do a lot of short trip that does not let the oil heat up? does the oil smell like fuel?

If it happen after a wide open pass you most likely have a pcv problem. This would happen from time to time on my old jet boat. Make sure your pcv is working well.