Welcome to the Two Wheeled Texans community! 
Feel free to hang out and lurk as long as you like. However, we would like to encourage you to My Throttle Position Sensor failed with the error C21 which is the sensor's Inlet Air Temperature element.
I set about finding a new IAP/TP/IAT sensor but Suzuki wont sell one and Mikuni didn't reply to my email. Copies of part number 90224500 are available on ebay for 13GBP from China and the listing usefully says that this sensor fits the Yamaha YBR125 and YZF125R .(Pre 2014 to be precise)
So, deciding to stay with a genuine Mikuni branded sensor, I picked up the throttle body from a claimed 10000 miles Yamaha 125 for 30 GBP from a breakers ebay shop, and simply swapped the sensors over.
I pinned the dealer plug to set the dash to the middle of the C, -C00. It stubbornly remained at _C00 but the bike appeared to be running fine.
Unfortunately it seems that my clever C21 Error fix was not so clever. I did a few hundred miles on local roads with no issues but I found the bike runs badly at speeds above 45mph.
I checked the voltage output of the TP/IAT/IAP sensor that I took off the Yamaha 125 throttle body, and with the throttle closed the output is low at 0.7v as opposed to the 1.1v listed in the Suzuki manual. From this I'm guessing the bike is under gassed with the symptoms kicking in at 45mph.
I refitted the original sensor, it was the IAT element that failed, and the voltage output was the correct 1.1v so I'm assuming that the power input to the sensor is OK.
I found the service manual for the Yamaha YZF R125 online and the output from the sensor with the throttle closed is listed as 0.63v-0.73v. So, at 0.7v the sensor is behaving as if it is still on the smaller bike.
My questions then are. Is it possible that having been used on a smaller bike the sensor output is now restricted? Or is the Mikuni sensor specific to a particular model?
I hope that makes sense. Best wishes,
I set about finding a new IAP/TP/IAT sensor but Suzuki wont sell one and Mikuni didn't reply to my email. Copies of part number 90224500 are available on ebay for 13GBP from China and the listing usefully says that this sensor fits the Yamaha YBR125 and YZF125R .(Pre 2014 to be precise)
So, deciding to stay with a genuine Mikuni branded sensor, I picked up the throttle body from a claimed 10000 miles Yamaha 125 for 30 GBP from a breakers ebay shop, and simply swapped the sensors over.
I pinned the dealer plug to set the dash to the middle of the C, -C00. It stubbornly remained at _C00 but the bike appeared to be running fine.
Unfortunately it seems that my clever C21 Error fix was not so clever. I did a few hundred miles on local roads with no issues but I found the bike runs badly at speeds above 45mph.
I checked the voltage output of the TP/IAT/IAP sensor that I took off the Yamaha 125 throttle body, and with the throttle closed the output is low at 0.7v as opposed to the 1.1v listed in the Suzuki manual. From this I'm guessing the bike is under gassed with the symptoms kicking in at 45mph.
I refitted the original sensor, it was the IAT element that failed, and the voltage output was the correct 1.1v so I'm assuming that the power input to the sensor is OK.
I found the service manual for the Yamaha YZF R125 online and the output from the sensor with the throttle closed is listed as 0.63v-0.73v. So, at 0.7v the sensor is behaving as if it is still on the smaller bike.
My questions then are. Is it possible that having been used on a smaller bike the sensor output is now restricted? Or is the Mikuni sensor specific to a particular model?
I hope that makes sense. Best wishes,
