So my bike an 08 US Versys has been surging and coughing at part throttle worst at about 2800rpm. Sure seemed like a lean stumble to me so i went looking for a cure. Lots of great info on this site but not a cure for this.
Removing the sub butterflies was not the solution though the improved responsiveness is very welcome.
So what it the fix you might ask, well before we get to that if you blow your bike up/void your warranty don't complain to me. Ok so there are 2 potentiometers on the left side of the throttle bodies, these are connected to the butterfly shafts. They are held in place with torqx screws. The top one tells the computer where the servo is holding the sub butterflies and fast idle cam. Since i don't have sub butterflies any more there is no restriction on the air flow when i open the main butterflies and no ability to close off the intake should the computer decide im going to fast, all it does now is set fast idle cam position. Rotating the lower pot shifts the fuel mixture curve as a whole leaning or richening the mixture everywhere.
I don't have the equipment to sniff the AF mixture but what i can tell you is my bike was WAY lean at 2800-3000rpm. So warm up your bike throughly. Then turn off the bike disconnect the plug on the lower pot,
the black plug its easy to get to. With a good multimeter check the resistance of the pot on the two outer pins write this number down if you want to go back to the stock stetting later rotate the pot to get this reading again.
Idle speed must be set the same when taking this reading and reseting it should you decide to, rotating the shaft by turning the idle screw will change this reading!!! Reconnect the plug start the bike set the idle about 2800-3000rpm with your torqx bit break loose the screw holding the lower pot in place and gently rotate the pot back and forth noting the effect on engine speed when you find the sweet spot where the speed is the highest lock the pot down and turn the bike off.
Dont take all day about this we dont want the bike idling at 3K for an hour! but a few minutes wont hurt anything either. You will probably notice the rpm went up quite a bit when you did this let the bike cool a minute restart reset idle for 2800-3000rpm and make sure the pot is in the sweet spot for highest idle lock the pot down reset you idle to 1300rpm and your good to go! Oh one more thing remember that top pot the on e on the sub butterflies shaft you will more than likely need to rotate this pot to set your cold fast idle unless you want to warm your bike up at about 4000pm
just loosen screw and rotate the upper pot to get the desired fast idle speed, i leave that speed to you to decide... For the two bike i have done this two the difference was huge millage does not seem to be hurt in fact the one that was coughing went up and best of all very butter smooth and no stumble surge!!! I suspect the mixture is not perfect everywhere, what stock fuel curve is? But i can live with a bike a bit rich on the top, i cant live with one coughing through the intersections...In the end this is much like adjusting a fuel screw... though this fuel screw changes the whole curve not just one area of it. Hope this help someone and sorry to be so long winded just wanted any one who tried this to have all the info they needed.
Best of Luck Chad