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Discussion Starter · #1 ·
I've read every old thread on TPS adjustment I could get my hands on, but I'm still a bit stuck here. Hoping someone might have some advice for me.

I am replacing what I think is a bad TPS sensor on my '06 Wee. Symptoms are generally stumbling/ low RPMs / stalling at idle after running for a while, but lately it's been happening with a cold engine, too, as soon as I fire it up.

I followed what seemed to be the general consensus from earlier threads:
  1. With old TPS sensor, ran bike at idle until warm, made sure idle was at ~1200 RPM.
  2. Swapped in new TPS sensor.
  3. Put bike in dealer mode, idled until warm again.
  4. With TPS roughly in the middle of its physical adjustment allowance (slotted holes), idle is still pretty much correct, but the dash is reading "_C00".
  5. I rotate the sensor in order to get to the middle C bar ( -C00 ), but as I turn it, engine RPMs are dropping lower and lower, and I am needing to adjust the idle RPM screw WAY high to keep up with it.
  6. When I eventually get that middle C, and my idle RPM adjustment is cranked way way high to achieve 1200 RPM, bike does not sound like it's running well at all, a lot of stumbling and coughing. If I give it a little throttle, it goes crazy - RPMs will keep ramping up instead of dropping after I let go of the throttle, and I need to shut the engine off before it redlines.
  7. If I adjust the RPM idle screw back to where it was before roughly, and put the TPS sensor roughly in the middle of its adjustment range, bike idles fine, and responds reasonably well to blipping the throttle, though RPMs do seem to "float" for a bit before returning to correct idle RPM. But the "C" bar reads in the bottom position again, _C00.
The situation I describe in step 7 is pretty much exactly where I was with the old TPS sensor. It was rideable, but some jerky throttle response at low speeds, and sometimes dying at idle. The new TPS sensor is a brand new genuine Mikuni OEM part.

Other things I've tried for this problem - ran the service manual checks on my alternator (good), got a new battery, and made sure my clutch lever ignition safety switch is working correctly.

Any advice would be greatly appreciated!
 

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Certainly sounds like a tps problem is plausible, though also sounds like the adjustment is not the core of the issue.

With what you have on hand now, a quick test would be to swap the tps and the stps and see if the symptoms change. If the tps is suspect, then you would also need to suspect the stps.

Below uses standard terminology. So it can be googled if you are not conversant.

Test the sensors.
The position sensors are 5k potentiometers. Pot for short. Armed with this info, you can google "testing potentiometers" for some hints. I use a mutimeter with a bar graph on the bottom. I think most meters have this. You can watch the potentiometer's response to the throttle position and if the response is jumping around, not nice and linear, then the sensor needs replacing. I took a little video of this checking my tps's for this website, but have not yet figured out posting video. So here is a pic, the bar graph is below the digital readout.
288651


Relevant info:
The wiper is not the center pin, it is one of the end ones. Use the meter to identify the wiper.
To connect to the sensor I used the female end of "dupont wires" - from aliexpress
They are a tight fit, care is needed not to bend the sensor pins.
You could also cut the harness and rejoin if you have no fast access to these little sockets.

288648


To be really clear about what is going on you need to setup your test to maximize the resolution of the bar graph. To do this you need to understand your multimeter. Do not use auto range.

If you use the 40k resistance range to test a 5k pot then only 12% of the bar graph is going to be utilized. Conversely, if you put 20 volts across the pot and set you meter to the 4 volt range then you can use all of the bar graph to examine the bottom 20% of the sensor's response. For the tps, this is probably the area most likely to wear.

Using a voltage applied across the pot gives more testing flexibility than the multimeter's available resistance ranges.

Volts applied to the wiper instead of across the pot may damage the sensor due to excess current when the wiper gets too close to the other connected terminal. If there is any chance of incorrectly wiring it, use a 1k resistor to limit the current to safe levels.

288649


Hope this helps.
 

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Discussion Starter · #4 ·
Thank you guys for the advice. The TPS sensor is a brand new OEM part out of the box, so I doubt it's the issue (especially when I had the same symptoms with the old sensor). I will double check the O-rings to make sure they look right.
 

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First take a look at the TPS that you took off the bike. If this one still has the O-ring around the stem, it's probably not an O-ring issue. But if this old TPS does not have the O-ring around the stem, well, it has to be somewhere, right?
 

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Discussion Starter · #6 ·
Took some pictures because you guys are saying the O-ring could be the culprit and I want to make sure I've got everything set up right.

The TPS assembly has 3 removable components, at least on my particular specimen:
290396

The sensor itself, the white plastic bit, and then the black rubber piece that fits within the white plastic bit. Is this the O-ring y'all are referring to? It doesn't seem like it necessarily seals against anything, so I wasn't sure if it qualified as an o-ring.

Here's what the inside of my TPS housing looks like with these three components removed - does anything look amiss? should there be an o-ring there I'm not seeing?
290398


Here are the two additional bits installed before putting the sensor on - do I have the orientations right?
290399
290400


Lastly, here's the sensor on, in the position it needs to be to register -C00 on the dash. You can see it's almost at the limit of its adjustment range, and the idle is still extremely unhappy (either dies or needs to be adjusted way higher than it should be, and then the throttle behaves wildly).
290401


Thanks for any further insight you guys might have.
 

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Do you have the service manual? For pre-2007 Wee's the manual includes the procedure to synch the butterfly valves, i.e. throttlebody linkage. If these are out of whack then it might make the TPS impossible to set correctly. Check the section on TB service and follow the procedure as the order is important (idle screw, TB linkage and TPS settings). Also, check for vacuum leaks.
 
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What is the restistance reading of your new TPS? If memory serves you are looking at roughly:
Throttle valve closed value: Approx. 1.12 k ohm
Throttle valve opened value: Approx. 4.26 k ohm
There have been a case or two reported of receiving the incorrect part. Compare the resistance and pin out values of the old vs new TPS, as well as direction of turn - TPS Compatibility Issue
Finally, you are waiting a few seconds between each adjustment change, for the ECU to register the new value of the TPS setting?
 

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Discussion Starter · #9 ·
Update on this thread for anyone that might find it useful in the future. Gave up and took it to a mechanic this weekend. He diagnosed the problem as water in the fuel tank. After draining and refilling the tank (plus a little seafoam), he was able to adjust the TPS sensor to the middle bar and maintain the proper idle just fine. It performed great on the ride home, so fingers crossed that's the end of it. I still suspect I probably needed the new TPS sensor to begin with, but wasn't able to adjust it properly due to bad gas.
 

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Thanks for the follow up. I hate reading through a thread and getting to the end and finding the OP never gave the solution and/or outcome.
 
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