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Hello guys, I am new to this forum and wanted to look for some advice on a starting issue I have had with my KTM Duke 390 to point me in the right direction, it is a 2018 model.
For a while, every time I try and start the bike, the starter motor seems to be lazy and doesn't spin over enough, it sounds really weak however as soon as I connect a external 12 volt jump pack the motorcycle starts instantly, and throughout the day the starter motor will function fine and start on the button every time as long as I have taken the bike for at least a 20 minute ride.
When trying to start the motorcycle I always have about 12.6 Volts however, I seem to have voltage but not enough amperage seems to be going to the starter motor to spin it over.
I have replaced the following :
Brand new battery.
Starter Relay / Solenoid (I have also tried bridging the terminals to bypass the relay and the motorcycle still wont start, I had to replace the solenoid because the last one failed and was causing a short which caused the loom to fry, I had to solder the whole loom and put some new earth cables on my bike, I have also cleaned all of the earth terminals with some abrasive.
New ECU- I have tried a different ECU that I know works.
I have tried charging the battery overnight but it still wont start without the jump pack.
Starter motor: I have replaced the starter motor with one that I know works, and it started on the button as soon as I replaced it. After I left it for 2 days and it was a cold morning it wouldn't start again and I am now having the same issue.
I am totally lost on what to do, if anyone could give me some advice and point me in the right direction I would much appreciate it.
Many thanks, Barney.
 

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A weird issue indeed.
Have you checked the kill switch and starter button?
Only thin I can think of next id to measure the voltage at all points involved with the ignition on only and while pushing the start button.
Good luck.
 

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Can you check the voltage at the starter motor when it's trying to start?

Also, try to turn the engine over first, say by rolling the machine a short distance in gear, and then try starting it and see if it starts.
 

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It seems either your battery is not charging enough. I guess you might have a faulty generator coil that isn't charging up your battery considering it starts fine with an external pack. Start off by inspecting the terminal voltage with the engine running at 5k RPM. The voltage should be around 15.5 max. If it's low or shooting up, your generator is at fault. Secondly, make sure the starter ground is right, a faulty ground or a rusted ground would mean it won't crank or crank intermittently. Try inspecting the above and see if it helps.
 

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Like Blackthorn said. Most starting/electrical problems are a corroded ground terminal( battery, starter, solenoid)
 
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