when at idle i have a occasional intake backfire. i really don't remember it doing this when i bought the boat and motor new. its a 90 hp 4 stroke 2000 year. it doesn't do it every revolution, about every 5 seconds, it will do a poof from the intake at idle. does ok at high speed. haven't had it on the lake, been running it in a tub of water. could this be normal? could it be the fuel, its been stabilized and is only a year old. could it be the ecm changing the ign timming at idle? a friend of mine says he remembers it doing it when it was new. plugs and wires are good. any thoughts?