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Need help diagnosing an issue

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Hi all,

Apologies in advance for the long post. We have been dealing with an issue for almost a year and its been a long road.

KTM - a European motorcycle manufacturer - released their first 2 stroke EFI dirt bike just over a year ago. I was first in line to get one. I struggled with the bike for quite a while as it had an issue with low throttle openings (cruise). It took me a while to figure out it was this particular bike and not a problem with other bikes of the same model.

The problem is at small throttle openings there is a stutter/burble which causing a hesitation, then a bit of a launch when the throttle is opened. It is erratic and unpredictable. For fast racing this is not a big deal, but for technical competition it is a critical issue.

After hunting around I found another 10 or so owners had the exact same problem, so we all got together and have been working to solve the issue. Unfortunately KTM dealers and the KTM customer support were of no help to us. They claimed “that is the way the bike runs”. I have to stress that 1000s of other riders with the same bike do not have this issue. I have ridden some of these bikes and was amazed at how well they ran.

The ECU on these bikes is locked down. No aftermarket diagnostic tools exists other than the torque app with a few basic sensor readings The dealer does have a tool but they (a) wont let s “work” with them” and (b) want $150 an hour to diagnose it on their own. Some of us have installed an aftermarket ECU, which has helped the problem, but has not cured it. Unfortunately their software is limited: no data logging, just live value reading.

Here are all the things we have tried:

- New park plugs, different gaps

- New ignition coil

- Slightly retard/advance ignition (2 degrees)

- Different fuel maps - leaning the bottom reduces pipe bang, but not issue

- Different fuel

- Fuel pressure testing

- Injector flow testing/cleaning

- Injector flow rate testing

- All sensors have been tested for proper range

- All sensors tested through the ODBII connection

- CCP (crankcase pressure) sensor testing

- Proper TPS adjust, reset TPS

- Idle stop screw properly adjusted

- Idle air screw

- Different heads (RKTek, S3)

- Reeds

- JD tuner

- Different Gas

- Different 2 stroke

We are now at a place thinking the issue may be a ground problem or a shielding problem. All wires – for the entire bike – all travel together in this harness. Unshielded.

The ignitor is onboard with the ECU

There is only a single ground coming out of the ECU, which goes to the frame. Sensors have their own grounds going back to the ECU.

The first attachment is a block diagram roughly showing the wiring layout to the various components.

The second picture roughly shows the ignition coil to spark loop - a possible source of EMI.


Thanks for reading this far.

Without logging, it is difficult to see what the ECU is doing when the problem occurs. So a lot of our attempts to fix the problem have been mostly a parts swapping exercise.

I feel confident it is a noise issue, but I have been confident before. I would appreciate any suggestions on how to diagnose this further.

As an alternative, I have removed the harness and think there can be some improvements made. I have attached a blokc diagram of the harness, a picture of the ignition loop, and picture of the harness itself. I think the grounds can be adjusted to reduce the EMI of the ignition loop and strategic shielding. Here are some questions with respect to the grounding and sheiding:

1. the main ECU ground is at the frame near the engine. Would it be better to have it go right to the head? I am thinking this might reduce the size of the ignition EM loop.

2. The battery ground is at the rear subframe. I would like to run a ground cable to the engine near the starter. Would this be a better option?

3. Since all the wires run in a single bundle I would like to shield wires. The uSquirt manual says to shield injectors and ignition together, but since I have so many other wires, I would like to do the opposite: shield the sensors. If I do this, where should I ground the shield? To the common ground point?

Attached Files

The tool to look for electrical noise is an oscilloscope. You should find a garage that has one and work with them on the diagnosis.

Since 1000s of bikes don't have this problem, then using a scope on one that does and one that doesn't should give you some clues.

I do have a scope and have used it, while running at idle on the CKS and ignition. However, I have a few challenges:

- the problem occurs while under load and it is difficult to take a scope on a bike.

- neither of the ECUs have data logging

- no dynos on the island

- no body is going to lend me there good running bike so I can probe into it.

Any thoughts on the grounding/shielding?

I think there are 1000s of reasons it's not a problem, but adding extra grounds, as well as making sure that there is no paint at the connections is a good idea.

Using your scope to measure the voltage drop across the various grounds can give you an indication of places that may not be making sufficient contact when under electrical load (like while cranking the starter).

OK thanks I will look into your suggestions.

I wholeheartedly invite others to comment on how we can diagnose the issue. It has been a long haul for a number of us and would happily invite help.