My mother has a 2006 suzuki s40. It is a single-cylinder 650 i think. Any ways, since day one, we bought it with half a mile on it and now has 4500 it has always back fired during deceleration. It doesnt do it all the time but something like this. If she gets up to 60 and lets the throttle off 5 times a day, it may backfire 2 or 3 times a day. Her friend has the exact same bike, same thing, bought it new with a mile on it. They both have taken it to the dealership and they say that it is normal for that bike. Something about because it is a one cylinder something something something. I personally dont not think that it is normal. First idea is to check valve clearance, When I did it the first time, everything was normal, Its bee 2,000 miles now and I will be checking again but everything at this point has checked out fine. I have checked the bike over and over again and cant think of anything else that would cause it other than valves. I am starting to believe that this is truely normal for that bike. Before I start letting it go, I want another opinion on it. Does anyone else have a s40 and share this issue? If it is not normal, what can be done to fix it?