This is regarding an argument I'm looking to settle with my friend, over in particular a Triumph TT600 motorcycle. When off the throttle and above 6k RPM, the engine will 'hang' at 4.5k-5k for a few seconds before dropping. He has a power commander installed and can edit the fuel mapping at will.

What I'm after is this: can an engine be 'too lean' at 0%/Closed Throttle? Is some fuel injection required to avoid pre-detonation or 'too lean' conditions? Or could I just use the power commander to remove more fuel from the 4k-6k, 0% throttle range safely?

Note: To settle the argument, references and/or a well reasoned theory will be required.

  • At closed throttle above idle RPM, normal fuel injection systems close the injectors completely... Hence the difficulty some modified cars have "catching" the idle on a high RPM drop to idle. It can drop through the fuel injector turn on point too fast. I know zero about Triumphs/motorcycles specifically though... Oct 7, 2013 at 11:28
  • Could you put this as an answer with some links for reference material?
    – Ehryk
    Oct 7, 2013 at 16:26
  • as much i know if the vehicle is on neutral and you leave the throttle still's little fuel go to engine to run it
    – Deiknymi
    Oct 7, 2013 at 16:40
  • The issue that was resolved with the TT600 was related to the fuel map in the ECU. Since you cannot burn a fresh ROM onto that without having some geek tech skills the easier route is to get a power commander and load the map for your bike with the stock exhaust. If you run into problems they provide live phone tech support from US based resources. This can smooth out the jerky issue and if you buy and aftermarket exhaust at a later time you can just load the map for your new exhaust and the bike is tuned. Dec 8, 2015 at 13:23

2 Answers 2


As Brian Knoblauch stated, fuel injection systems normally shut off the fuel delivery completely when the engine is in an "overrun" state, i.e. when you get off the accelerator completely with the vehicle in gear and the RPMs sufficiently high. In this sense, you can't be too lean in overrun--it's perfectly acceptable to have no injection at all.

As for this specific application: it's possible that the fuel injection controls do not turn off fuel delivery on overrun. Sometimes motorcycle control systems are designed so that engine braking is reduced to enhance stability.

That said, the 2000 TT600 was apparently known for bad on/off throttle transitions and funny throttle response. They made changes in 2001 and 2002 to attempt to address these issues.

  • As for reference material, check out Bosch's "Gasoline Engine Management," its a fantastic book, and judging by your question, I think you might find it an interesting read. Lots of good theory.
    – mac
    Oct 9, 2013 at 18:16

Throttle never reads 0% as the throttle plate angle is around 25~30 degree's. RPM should not 'hang' on a fuel injected bike, you may have leaking injectors or fuel pump regulator may not be working properly? Your much more likely to get after-fire in exhaust system on over run. What year is the 600? I know early ones had some issues but all the newer models can download/install new maps either direct from Triumph or various after-market and Triumph forums. I think the Power Commander may be your problem, some bikes have issues with the inputs/outputs. I'm a lot more familiar with Suzuki but I have a friend who has Triumph 600 and was a Triumph tech before he started teaching Honda.

  • It's a 2000 Triumph TT600, the first year of Fuel Injection. How would I test the injectors or FP regulator?
    – Ehryk
    Oct 9, 2013 at 17:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.