So my bike has a high idle (gone up as far as 6k revs sometimes). There are no air leaks and the throttle freeplay is fine and cables all nice and free. Clutch is fine also. The fuel lines and injectors should be nice and clean as I have used some redex in the last 2 tanks. The idle can be fixed by resetting the tps but this only lasts a week and then the problem is back.

The bike is a Honda Grom MSX125. What could be causing this? Cheers.

  • 1
    How sure are you about the absence of air leaks? Does the engine stall if you cover/seal off the intake? – Zaid Apr 2 '17 at 11:13
  • I have checked around the intake manifold and exhaust and there doesn't seem to be a change in rpms that would suggest a leak. Also resetting the tps fixes it for a week. There are no error codes suggesting that the tps or other sensors are faulty or malfunctioning. – DamoC Apr 2 '17 at 12:26
  • If I fully close the air screw it sometimes dies and sometimes keeps running. I know that could suggest a leak but why would it only affect it sometimes? Cheers – DamoC Apr 2 '17 at 12:28
  • When you close the air screw, does it run when the engine is cold or hot? Also, it might be useful to know which Honda this is – Zaid Apr 2 '17 at 12:48
  • If I remember correctly it doesn't matter if it's hot or cold ( but mainly tested when cold ). Yes sorry, It is a Honda Grom ( MSX125). – DamoC Apr 2 '17 at 13:12

This problem was resolved by replacing the TPS.


Check the throttle body inside maybe you have to clean it check the temperature valve of water maybe you need a new one
Or you have to fill up some water to the water tank


There is definitely a significant leak somewhere. At idle the throttle plate should be almost closed with only a very small gap for mixture to pass through. With this high vacuum there is no way that the engine would rev to 6000 rpm. I would look at the rubber intake manifold and also the throttle body itself to confirm the plate is closing correctly and secure.

  • Definatley no air leak. It is very intermittent although resetting the tps sorts it. The tps is also fine though. The issue is definatley electrical related although I understand what you are saying. – DamoC Aug 16 '17 at 14:34

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