Originally Posted by fredsprint
I just want to understand how the return to idle is managed on Kehin version and especially 1050cc.
OK, you mentioned Sprint GT so I assumed you were trying to correct known stalling issues that appear to have been resolved by Triumph.
I don’t know if I can give you the detailed information you want but I’ll share what I know and maybe you’ll find some new information in there.
As you know the Keihin system on our 1050s uses a stepper motor that acts directly on the throttle cam to control starting, warm-up and idle. Keihin documentation also states that it controls “overrun correction”. This is related to your question but the documentation does not provide further explanation about what corrections it makes. The stepper motor can only control the throttle at small throttle openings. Its range will give a TPS reading of 0.60V when the stepper is closed to 0.75V when the stepper is fully open. Compare that to throttle position which reads 0.60V when fully closed up to more than 4V when fully open. My 1050 Sprint gives a TPS voltage of 4.1V when the grip is twisted to the max but this figure varies from one machine to another. It shouldn’t go higher that 5V since that’s the voltage sent to the TPS sensor.
When the ignition is off the idle stepper is closed. The control lever arm is not even touching the throttle cam. In this position the throttle return spring can close the throttles completely. When the ignition is turned on the ISC Stepper Motor moves the control lever arm to push against the roller on the throttle cam. This opens the throttles slightly for starting. Once the engine is started the stepper motor will move to maintain idle according to coolant temperature. As the engine warms the target idle speed is reduced. For my 1050 Sprint target idle speed is 1170 rpm at normal operating temperature. This is set in the engine management software but can be changed with TuneECU. The ECM compares actual engine speed with this target speed then signals the stepper to move to maintain target idle speed.
OK, so that describes normal operation but what happens when you open or close the throttle? Well, I don’t have any official information so I can only give you my thoughts. If you twist the throttle slightly for a small increase in rpm then I would guess the stepper will close slightly trying to reduce the engine speed. Of course this has no effect because the throttle cables are holding the throttles open. I think the stepper will stay in that position until you release the throttle grip and the rpm reduces back to idle speed. Then the stepper will move again to maintain idle. There is another possibility – maybe when throttle opening goes above a certain value or the rpm goes above a certain speed the ECM tells the stepper to move to a “normal” idle position ready for when the throttle is closed. I have not watched the stepper in that situation to see which of these happens but my guess is the first situation applies.
If your friend is having trouble with poor return to idle and stalling then the first thing I would check is throttle balance. If he's done that already then obviously he should check TPS and ISC Stepper Adjustment
. If the readings are correct then the stalling is caused by something else and trying to fix it by adjusting the stepper away from the correct settings could result in poor starting.