Hi everyone, this post is to share my experiences with my bike stalling in neutral. I believe I have solved the problem with my bike, and I hope my experience can help others deal with the same problem.
When I was trying to search for the problem on google and youtube, most of the results were pointing in other directions, so I hope other people will have an easier time finding this post.

TL;DR
The bike could start but couldn't idle. A loose sparkplug connector prevented the engine from idling. Reinstalling the sparkplug connector initially seemed to solve the issue.
Later, the problem seemed to reemerge; the bike would sporadically stall while idling, though not as severe as before. Appears the previously poor sparkplug connection caused the ignition coil/sparkplug connector assembly to fail, resulting in misfires, hesitant idle and stalling. Replacing the ignition coil/sparkplug assembly solved the issue. The bike starts and idles just fine now.

Background
I bought my 2017 RC 390 second hand in 2020, when I first got the bike, it would stall while cold, the seller said this was common with RC 390s and I believed that.
Over the next few times riding the bike, the bike would barely idle, I would have to immediately hold the throttle open a little after starting to prevent the bike from stalling. I would try starting the bike in neutral, with and without the clutch pulled in, with the kickstand up and down, and also tried waiting for the fuel pump to prime a second time after switching the key on, I saw no difference in its ability to idle. I put off riding and working on the bike due to work but recently found the time and energy to address the problem.

The problem
Trying to start the bike would result in the engine starting and running for about one second then immediately stopping. Occasionally, the engine would run for long enough for me to rev the bike and hold the throttle open a little to let it idle higher. It seemed at the time that letting the engine warm up would let the bike idle more smoothly, but this issue worsened over time to the extent where I would not be able to even let the engine run for long enough to let it warm up.

Online research
Researching the issue on google and youtube yielded the following results:
  • Dirty throttle body and injector.
  • Clogged fuel/air filter.
  • Faulty clutch/kickstand sensor.
  • Worn sparkplug.
  • Live with the stalling issue/install piggyback ECU.

Trying the solutions
Cleaning the throttle body and injector seems like the cheapest first step, but I'm lazy and doubted the efficacy of this solution, especially after watching this video (
), the poster seemed to be experiencing the same issue and they tried cleaning the throttle body and injector only to find those parts to already be clean and even after cleaning those parts, it did not solve the problem. In the end, the dealership claimed they fixed the bike by replacing the fuel filter and injector.

I thought to try the easiest solutions first, so I started with replacing the fuel and air filter. The old filters looked pretty reasonable but I replaced them anyway, this resulted in no observable difference.

Then I tried replacing the sparkplug. The first thing I noticed while replacing the sparkplug was how easy it was to remove the sparkplug connector, and it seemed like the rubber cap on the sparkplug connector was not secured in place. I didn't see any soot in the sparkplug connector, but the terminal on the old sparkplug looked slightly discoloured. I then replaced the sparkplug and when reinstalling the sparkplug boot, I made sure to press the connector onto the sparkplug terminal properly, there is a click when the connector is installed properly. I also made sure the rubber cap was secured properly as well. This solved my problem, the bike started and idled just fine afterwards. Doing some research about the loose sparkplug connector, this doesn't seem to be an uncommon issue, but I'm not sure why I didn't find this solution earlier but I supposed most people would know to check the sparkplug connector before jumping on the internet.

After riding the bike a few times, the problem seemed to reemerge, though not as severely as before. The bike would start and idle for maybe thirty seconds before stalling, long enough for me to hold the throttle open to warm the engine up, though engine temperature didn't seem to make a difference with the stalling problem, in fact, a warm engine appeared to stall just as, if not more, easily. I could hear the engine misfiring at idle, and the misfires worsen until it stalled. I thought perhaps I didn't reinstall the sparkplug connector properly, or it may have come loose again, so I checked the connection but it seemed secure. Searching on youtube brought me to this video (
) about ignition coil failure on a Duke 390, and doing some more research on ignition coil failure (https://www.aa1car.com/library/ignition_coils.htm) this seemed like a plausible cause for my problems, especially considering the previously loose sparkplug connector. I replaced the ignition coil assembly (part no. 93339006033) and it seems like that has solved my problems (for now).

The strange thing is I measured the resistances of old and new ignition coil assembly and they are basically the same. The secondary resistance on the old coil was approximately 4.6 kOhms, and the total resistance of the sparkplug connector and wire was approx. 1 kOhms, this should result in 5.6 kOhms in total resistance when including sparkplug connector in the secondary resistance measurement. This 5.6 kOhms is the same as the new ignition coil assembly I bought. So I guess I either screwed up the measurement or in this case ignition coil resistances is not a definitive test for ignition coil failure.

Conclusions
A loose sparkplug connector led to engine stalling and ignition coil failure. Replaced ignition coil assembly and the problem is solved.

It appears that stalling issues on KTM 390 bikes are not uncommon, though my perception of this is probably because that is all I have been searching for on google. I recommend those experiencing stalling issues while idling in neutral to check their sparkplug connectors for proper installation and contact, and if there is evidence of a faulty connection, keep an eye out for ignition coil failure.
 

Icehax

Member
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Had i found this post a few months back when i had the same problem it would have spared me a headache. If anyone is having stalling problems first thing check that the sparkplug connector is properly seated, mine was halfway in, the bike would start but i would skip a spark sometimes making me think all kinds of crazy ideas... thankfully by the time i figured out the problem the coil was still good.
 

rajeev4248

New Member
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I am facing similar problems but in yotube I have seen some videos showing its a stepper motor problem and some are showing its a coil problem which one I should check for ?? Probelm is same as you explained
 
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