My 2013 S40 runs flawlessly at low to mid RPM in any throttle position, but acts like it's starving for fuel as it revs under sustained wide open throttle (like when winding out 3rd under hard acceleration, or 4th at 60 in wide open throttle).  
From low throttle load to sudden downshift and high revs, no symptoms for 5 secs, then surging and hesitating, like when it hits reserve at 55mph.

Diaphragm in carb is brand new (replaced because engine was pinging badly.. pinging is completely resolved now.)  
No changes to any carb settings or needles or exhaust or manifold, spark plug is new, air filter is new, valves are on spec. Bike is mechanically 100% stock.

Carb was completely clean when I disassembled it.  Jets are all clean, fuel filters in petcock and at needle valve were perfectly clean.
58K miles of flawless riding (running ethanol free premium) on the bike until recently.

Rectifier was replaced 200 miles ago (was charging 14.8v & cooking bulbs.)

What makes for bucking & sputtering, only at WOT high rpm?

I tried running on PRI.  Same symptoms there.

Try loosening the fuel cap....maybe the vent in the cap is clogged.

Dave wrote on 11/26/24 at 02:16:55:


Just curious how one would do that with the locking fuel cap? Mine only goes in one way. Not a twisting cap.

If you're ripping and it starts bucking, if you back off the throttle, does it start running right?
If so, then it would be good to get a second key and get it messing up, and open the NOT EVEN Almost full gas tank.
Actually, if you're down almost to Reserve, the tank is mostly air.
And you open it up and in seconds, it's bucking.
You fill up,open it up and, same amount of Time passes,and it's bucking
It's not the cap.
It takes a while to use enough gas to make the tank a low pressure zone when it is low on gas, a,lot longer than when it's full.