[macshpman]

I had a friend with a boat that his vent line to the fuel tank had become plugged by a insect building his home inside the line. The tank would build a vacuum causing the fuel system to lean out, and stalling backfire would be the signs of the trouble. It would act differently depending on the amount of fuel in the tank and how long and hard he ran the engine. Its worth looking into for sure. He chased this problem for a summer and was changing parts like he was changing his clothes. Very frustrating for him to not be able run 30 miles out from shore to do his fishing and not sure if he was going to be able to make it out and then back in.