Hanging Idle - Not necessarily the jetting

I have been doing a little digging and found that the hanging idle can be caused by the throttle position sensor fighting against the the manual throttle setting. This was the case with my bike and I had been getting a lot of information stating that it was probably the jetting. The throttle position sensor has three wires, yellow, blue, and black. Disconnect the throttle sensor from the harness (located under the fuel tank). The bike should idle just fine with this disconnected if it is the throttle position sensor. You can't just run the bike without this hooked up as the throttle response will not be good. Measure the resistance between the black and blue wire it should measure between 4-6k Ohms. Record the actual reading, mulitply it by .13 and .15. Here's an example 5kohms (x) 0.13 - 0.15 = 650 - 750 ohms. Now take and check the resisitance between the yellow and black wire of the throttle position sensor. If the resistance is out of spec. loosen the torx fastener that holds the throttle position sensor tight and rotate the throttle position sensor until the Ohmeter shows the resistance you just calculated. My measured to actual was off considerably.

so you use the resistance between the black and blue wires, and the figure you get, use that in the formula to get the resistance you want between the yellow and black?

if you move it to set the resistance correctly, does that not also change the resistance between the black and blue? then youd end up always gettimg a different variable to plug in the equation and always need to readjust to set the yellow-black wire to that new figure?

of course if the resistance between the blue ands black never changes no matter where the TPS is set at then what I just babbled about doesnt matter at all.

Remember when your dealing with resistance in those ranges temperature can make a big difference in your readings. Good tip though, I've always set them by half tapping into the connectors and reading the voltage.

The resistance between the blue and black should be constant regardless of the movement of the TPS. This give you the base line to calculate the range of the at-idle resistance as each TPS should vary between 4-6 and with the multiplier of .13 to .15 there would be a significant difference when you get to the ohm measurement you need to properly set the TPS for idle state. The carb on these things are an absolute female dog to get out of the bike......even when you have the top end pulled. Please note to recheck the yellow/black resistance after you tighten the torx bolt down on the TPS as it doesn't take much movement at all to change the yellow/black resistance. If you are having hanging idle there is a good chance that it's the TPS out of adjustment. I love technology......even when it comes from Electrical Engineers. I'm an ex-mechanical engineer now in sales and the majority of the EE's were a wacky bunch.....not all......but most.

A hanging idle can also be traced to to tight a return cable on the throttle. Back it of and away it goes. May not be your prob, but could be sombody elses.:applause:

Fitzy...good point...this is true.....sometimes it's the simple things that are overlooked.

ploflin1, did this cure your hanging idle problem?


It helped it considerably. When the bike is cold it still seems to run a little high and then when I blip the throttle it comes right back down. My throttle position sensor was off a pretty good amount but then I adjusted as noted in the service manual while I had the top end off for a rebuild. It's easier to get the carb out with the top end out but not by much. I would certainly add checking the resistance when experiencing a hanging high idle situation. The connector that you need to check is under the tank zip-tied to the right side of the frame.

Cool, Im going to give that a try, now did you check it with the motor runnung or while off? I would think you would at least have the motor warmed up as to bring things to operating temps?

I didn't have it warmed up as I had the engine dissassembled doing a top end rebuild. The resistance when measured can fluctuate a little due to temperature differences but it should not be enough to make a difference. Here is a good definition/explanation of what a TPS is and what it does.

A throttle position sensor (TPS) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the butterfly throttle valve.

The sensor is usually a potentiometer and therefore provides a variable resistance dependent upon the position of the butterfly valve (and hence throttle position).

The sensor signal is used by the Engine Control Unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

If this thing is out of adjustment it can change the timing and fuel mixture thus making the engine run odd such as a hanging throttle. I just happened to read about this somewhere else, have a volt/ohmmeter, and since I had the head and cylinder missing I figured.....What the heck.....let's check it out. Sure enough it was out of spec. A quick loosening of the torx screw a rotate of the TPS one way or another while watching the volt/ohmeter and tahhhhh dahhhhhhh..........it's fixed.

The only problem is getting to the carb............what a Bii-otch (in honor of Wii). It was a royal pain in the RRRRR's and I had the complete top end out. I can only imagine take out the poop shoot side.

If mine ever needs it again I may buy a Schwinn.

I checked mine (and I am going to check it again) but I had to crank it to danm near the end of the adjustment range, I mean it's way off. could the TPS be bad all together? did you have to make that large of an adjustment also?

this post is way more technical than I can follow but I have seen a hanging idle many times caused by an air leak in the carb boot.


As long as you can adjust within specs I would not worry too much.

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