Yes, I can get the bike to accelerate up to 65 if I use the throttle slowly and ease it up to about 1/2 to 3/4. Slightly above 3/4 is where I get the sputtering. When you have the throttle wide open from say 35 mph or less, it will go up to about 45-50 before it starts sputtering when you still have the throttle open. It cause the bike to shake and hesitate like you are running out of gas. Backing the throttle off until about 3/4 or less cause the motor to even out and you can then increase the speed to about 65. I don't wan to get a ticket so that is why I back off after 65. I believe it is a fuel issue but have done the tests that other s have listed, vacuum hose, primer, etc., and still the same. I did put in the Sea foam and about an ounce of Gum out Carb cleaner. It seems a little better but the sputtering and bucking like you are running out of gas is still there. I am think maybe I need to take the carb off and do a cleaning like Versay suggested. Maybe then I can se what is going on. Seems like I am getting fuel from the tank, and no gas is in the vacuum line so I am not sure what else to do other then take the carb out. If anyone has any other suggestions please let me know. I will let everyone know how I make out the my ordeal. Thank you. S40 Guy.

Vers, I didn't blow out the jets. I did not take the carb off yet. I cleaned up everything around the carb, tightened the ban that was loose that connected the carb to the cylinder, checked the vacuum line from the petcock to the carb and no fuel was in it, ran with prime and no change. I had not removed the carb yet. Everything on the bike should be stock as the person before me said the only thing he did was oil change, and looked like he put a battery in it. The batter was a maintenance free but was too large and I replaced that battery with the correct one so I could get the cover back on the Tool box holder. The sputtering when it happens keeps you at the mph you are at. It will sputter and spit for a long time and you can't go any faster. back the throttle off until about less than 3/4 and she takes off just fine. Maybe that is the way it is supposed to work on the S40, I don't know. When I do take the carb off how do I know what size the jets are? Do I have to replace the float bowl gasket? if I do I need to get one before I take it off. Thank you for your help. S40 Guy.

I think it's time for you to open it up and take a peek.
one note of caution, if it's the 1st time, those screws will be a bear to get out.  Put penetration oil on the threads and use a small impact wrench.  Support the backside of the boss or you'll break right off.
last ditch effort use a vice grip on the head of the screw.

Well put about 200 miles on her today and she is much better than yesterday. I can get 3/4 and a little bit more before the sputtering starts so I think I can live with that for now. Still have occasional back fire on decelerating, but again I can live with that. Most Harleys that go by are LOUDER!! than my back fire. I got this bike used to try it out to see if I would like it. So far I really like the S40. I think when this one comes time for major repairs I would look at a brand new one of these guys. so far I have owned a 2006 Kawasaki 125 eliminator (First bike after about 25 or more years of riding absence), A 2006 Suzuki GZ250, A 1995 Kawasaki Vulcan 750, a 1999 Yamaha Virago 535, and now my 2006 S40. My wife rides a Honda PCX150 Scooter and the Virago was the last bike we got for us riding two up on. Now that she rides the scooter, I wanted to go down in size to a smaller bike. yes, I know the S40 is a 3650, but the size, seat, handlebars, and overall feel of the bike feels smaller to me than my Virago. I considered an S40 or a Yamaha Virago 250, and found this one up for sale so we bought it. Even with the Throttle issue that is now livable, I still like it better than my other bikes. Not sure if the Gumout or the Sea foam, or just plain good gas running through took care of the carb issues or what it was, but I have ridden it today and had no problems at all going from 0- 60 mph fairly quickly, nor passing when I had to. Thanks to everyone who helped me out. If I was younger I think OI would tackle to get it near perfect. But at my age, I would rather ride and enjoy rather than keep fixing. Thanks again guys and I will keep you posted on how thing go. Take Care. S40 Guy.