My 2006 S40 is running almost perfect. I am running into an issue when I go full throttle. When I start out from about 30-35 mph, it will go to 55-60 pretty fast and no sputter or surging. Do the same thing from about 45-50 and it will surge and sputter until you back off about just under 7/8 to 3/4 and it will run fine up to 60 mph. Bike has about 11,800 miles on it. I did a new petcock, vacuum hose and fuel hose (yes, I got a new stock S40 petcock not a raptor). I had trouble with backfiring and did not seem to have much power at lower speeds so I turned the air mixture screw out or counter clockwise about 1- 1 1/4 turn and that seemed to cure the lower speed lack of power. The only problem I have now is at full throttle. I was thinking that I may need to remove the carb and maybe replaced or clean the jets. I know someone posted that the problem may be with the main jet and the Clymer book said the problem may be with clogged main jet. I really don't want to jet bigger size. I live in Indiana so you have an idea of altitude. I have read about a spacer mod, but could not find anything on what it really is and what it does. I know the previous owner did let the bike sit over a winter or two in a garage, but am not sure of stale fuel or not. When I replaced the petcock, it was all orange on the plastic. I also opened the float drain screw and removed fuel but it looked clear, no debris in it. When I did the petcock, the tank also seemed pretty clear, no rust or debris. If you keep it at 1/2 to 3/4 throttle it will get to speed with no sputtering. It only happens at full throttle. Also very little back firing when I decal unless I go from fast to quick out on the brake then I get a small pop. Any ideas or suggestions? Thank you. S40 guy

In the tech section you will find a trouble shooting guide, an indexed table of contents, and a companion CD of pictures that can be downloaded.
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The white spacer mod changes the height of the needle on the carb slide.  If you ever dealt with older carbs you will remember that the needle once had notches and a clip.  The needle could be moved up or down to make the engine run richer or leaner.  The white spacer modification simply involves removing the white colored spacer on the needle and either shaving off about a third of its height or replacing it with something thinner like a couple small washers.  (see the carburetor part pictures in that companion cd download, they show the spacer and one cut down to half height) This will make the engine run richer in the mid-range.  That is not your problem now but after you get your current issues resolved you might want to consider doing it.