I have a 2006 S40 Bike I bought used. At about 45-50 mph with throttle wide open it sputters and hesitates like it is trying to get power but cannot. The front band between the carb and the cylinder was loose so I tightened that and it did not help. The bike did have some old gas I n the tank because it smelled old so I put in sea foam with a full tank of fresh gas on top of the old. When you ease the throttle, it will get up to 65 MPH with no problem. it is only on full throttle that I get the sputtering. Also it will back fire a couple times coming back down from WOT. Any suggestions or ideas? Could it just be that I need to run a couple of tanks of fuel with sea foam to clean out the carb? Side note when I get the sputtering and pull out the choke as a test, the engine does not change at all. maybe the choke might be messed up? Any advise would be helpful. I am very disgruntled with the S40. Please help. Thank you. S40 Guy.. :'(

OK. If I try that and it works what will that tell me? I put in a new standard NGK plug so it has a new plug in there. Any help would be great. I am frustrated. I will try the prime. And re post the results. Thank you. S40 Guy.

I'd do a thorough cleaning 1st

Check yer' air filter, too...

Sounds like a fuel delivery issue to me.  WOT requires a lot more fuel than half or 3/4 does.

Could be a coil breaking down at high speed.  But, if it'll run the same RPM slowly going to 1/2 or 3/4 it rules that out and puts it back to fuel delivery.  Sounds like you're running out of gas.

Don't know what kind of mods if any are on the bike.  It might be that you have to rejet for something that has been done to the bike.

Been doing it since I first rode the bike. When I test drove I was in a parking lot so I only could use low throttle. There are little less than 7,400 miles on it. it was outside most of the time with a cover on it in Indiana, and during the winter it was in a garage. To the best I can see no mods were done to it. I did drain the carb and the fuel looked very clean. No debris. I was surprised. nothing bad at all in the clear container I used. Gas did smell a little stale. it had about a quarter of a tank when I bought it. I filled it up with my lawn mower gas (mid grade Phillips 66) and I had to wait to get two tires and fork seals done on the bike before I could test drive for real. Friday was my first day and it started the sputtering right away on WOT ONLY!. Yesterday I have the same results but not sure if it improved slightly or not. Around 45-50 mph is when you get buck city on WOT. I changed the oil to Spectra 10w 40 Semi Synthetic (same I used in my Yamaha Virago) when I got it, Changed oil filter, Put in new spark plug, and a new fuel cap as the one that was with the bike had the key broken off inside of it. Around the top portion of the tank there is a little rust near the top, but inside the bottom lip of the fuel tank it is clear as the rest also looked clear, so I assume the rust is only at the top portion of the tank. No rust flake, just the rust when you wipe a rag or paper towel on it to clear it up. Other than the WOT issue, and the occasional back fire (which I think it from the same WOT issue) the bike runs great. No cam noise yet but I am keeping an eye on it. Hoping for at least 12,000 miles or more before I have to get with Versay on a tensioner. I have 3.5 oz. of Sea Foam in the tank yesterday to run it through to help clean up things in the carb. I am an average mechanic with very strong electrical skills and average carb type skills. When I replaced the spark plug I did lose the speedometer O ring but got two new ones from Bike Bandit with the help form you guys here. No where does it tell you (owners manual or Cylmer manual 1986-2012 edition to watch out for this little guy to fall out. never even mentions this little guy. Any advise or help would be appreciated. Thank you for your help. S40 Guy.

It's a vacuum operated slide carburetor. You won't have instant response like we had on real versions with either a cable operated slide or a butterfly plate. And keep in mind, these bikes at lower elevations desperately need a larger main jet. Go up 1 size on the main jet, should alleviate the worst of the stumbling.

You're trying to get Porsche 911 Carrera throttle response out of a VW Beetle... in stock form, it doesn't happen.

Rereading the original post - This bike does not have a choke. It has an enrichener. Unlike a true choke, the enrichener does not work if the throttle is much above idle, so you can't really expect much change if you pull it out with a wide-open throttle.

Many of us seem to confuse WOT and high rpm. They are not the same, and in many ways are independent of each other. The original post describes a sputtering and hesitation with WOT, but at only mid range on RPM (45 - 50 mph). With part throttle the bike runs up to 65 mph. This could point to either a fuel delivery problem, where fuel flow is inadequate to support WOT demand, or to an ignition problem where WOT causes high cylinder compression pressures and something in the ignition is breaking down and not delivering spark. I would suggest checking the fuel flow first, partly because of the description of old gas and partly because it is easy to do with the bike stopped.