Originally Posted by JimN
Without being able to actually hear it, I would have to say that it's possible for the timing to be off enough to backfire and not be a hard "typical backfire" sound. It's hard to describe this kind of sound but I haven't heard a lot of motors produce the sound you mentioned. Still timing is something that should be checked if it hasn't been, yet.
I only checked timing by slightly rotating the distributor cap while the boat was on the trailer on the boat ramp in gear and out of gear. I got it positioned for the best running as I could.