Simon: Sorry, but this sounds more like a base engine issue. Can you duplicate the backfire? If so, put a vac gauge on the intake to see what it's doing. You may catch a flutter at idle, which means a valve is hanging open. How long has it been doing it? How many hours on the unit? I suspect maybe a bad lifter or something of that nature. That's why the lighter the oil the better - even 5W20 or 0W20. Never more than 5W20 on these engines.