I am the new owner of a 2007 S40.  The bike idles great and seems to do fine under hard acceleration.  The problem is that when riding at a consistent speed (partial throttle), the bike sputters and jerks.  It runs better with the choke on.  I have run it for quite a while to make sure it's good and warmed up the it makes no difference.  I have changed the plug, checked for vacuum leaks, checked and cleaned the air filter and this week I pulled the carb apart and gave it a good cleaning.  No change...  The bike has 7,500 miles on it.  Any ideas??

Thanks in advance

How partial is the throttle?  1/8, 1/4, 1/2?  What speed, what gear?

I'd say I'm at about 1/3 throttle.  I'm sure it's not a result of being in the lower gear.  It happens constantly when just cruising down the road in any gear.  I really don't think running it along at high RPM's is the answer either.  That can't be good for the engine!

Specifically,... don't cruise in 5th gear at less than 45mph..
                                           4th gear at less than 40mph..
                                           3rd gear at less than 30mph..
40mph in 5th is lugging...

I'm sure it's not a lugging problem.  It really is only good when I'm hard on the throttle, any other time it's jerking and missing.  This is not my first bike so I'm confident that I'm at the right rpm's in the right gear!

Thanks for all the advice, keep it coming...

white spacer mod

Arrite,, well,, thats just a good startin spot,, dont want to go into the carb to fix a problem that a wrist adjustment will fix. Now,, on to the fix,,

Yea,, what they said, time to go in & play with it, Getting those 4 screws outta the carb can be a real booger, be very careful lining up your tools.

I was just having a fit trying to get the rear turn signals off,, Took a #2 Phillips to the grinder & took the pointy end down till the + part filled the + in the screw heads at the top,, & it worked out just Fine!,,

Zachap,  time that you post that you tried out one of these suggestions that was told to you and what effect it had (or didn't have) on your issues.

You are narrowed down to the carburetor section of the bike.

Now it is your turn to tell us exactly what you did and what effect it had.

While you are getting ready to drop the bowl, please check your air filter for being all butt fugly blocked up and not passing enough air to properly feed the motor.  This isn't super likely because you run OK at full wick, but you gotta go into that area anyway and it is an easy enough check to do.