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I have an air cooled single cylinder motorcycle with a bogging down(feels like something is pulling back on the bike making it struggle) problem on acceleration. After many many months of having it I can now pin point the circumstances in which it happens.

  • It is throttle position specific (roughly 1/4, after this it runs fine).
  • It only happens when the engine is up to operating temp (on a cold engine it is fine(first 5mins of riding are problem free))
  • Trying to reset the ecm doesn't work (hold throttle 100% open then let go( almost as if the ecm isn't reading it WOT from the tps)
  • It doesn't matter what speed I'm going or what the rpm's are at 1/4 throttle it happens

The only sensors on the bike are:

  • Throttle position sensor (tested with multimeter and there is no spikes or jumps in voltage when slowly turning throtle, nice and smooth as it should be)
  • Intake air temperature (seems to be fine) Engine oil temperature (seems to be fine)
  • O2 sensor (not tested)
  • Idle air control Valve( ok not a sensor but still pluged in haha)

I have ran redex through multiple times to clean the injectors, tried different petrol and I'm ruling out fuel pressure due to the fact that surely after 1/4 throttle the demands should be higher and effects of bogging be even worse.

I have also tried air intake cleaner(the spray kind) to de-coke the throttkle blade.

The valves are in spec.

Changing the air filter and spark plug makes no difference.

All this happens underload but I tested in neutral (held the throttle at demon position if you will) and I swore I heard a few pops(backfires) although I can't be 100%(I'd say im 80% certain) certain due to the exhaust noise.

Finally the are no DTC's CEL's being thrown.

What else can I check? Im all out of ideas now.

Cheers and thank you if you made it this far


The day after this post I decided to go for a ride with the tps unplugged and as expected there was a delay between throttle twist and acceleration, but the 1/4 throttle bog wasn't there.

  • When checking the throttle position sensor you need to use a digital recording oscilloscope. The kind of jumps or glitches that would occur are far too fast for a multimeter to capture. – vini_i Sep 26 '17 at 17:59
  • Unfortunatley I have no access to an oscilloscope otherwise I'd be straight on it. – DamoC Sep 27 '17 at 15:41

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