When I hit the start button it will do one of the following.
Start immediately with no problem.
Turn the engine over very slow to the point of my trip odometer resetting. (Battery can't supply enough voltage and lights go out)
When this happens, I wait a couple of seconds and again it will either start immediately or turn over extremely slow.
New Ford Solenoid to handle high current.
Bike runs great.
The best chance of starting when the bike is in neutral.
Start switch is working because the starter relay is energizing and the starter current does not go thru the switch. Intermittently there is not enough voltage/current to the starter motor, the voltage drops trying to supply enough current and the odometer resets from the low voltage. You have a bad/weak/loose connection at the battery terminals + or -, or your ground to the frame/motor is poor. BE SURE your battery terminals are tight! I know there is a ground fix for the 1800 but I'm not familiar with that. Where your ground cable connects could be an issue.
05 1300R -sadly sold 04 BMW K1200LT - Luxury Tourer, smooth efficient quiet power,sporty mile eater 05 BMW F650GS - rugged and elegant adventure tourer, does it all well, high speedcruising to rough dirt roads IBA: SS1000,SS2000, SS3000, BBG1500, 50CC, UCC, LGL1000, 4 Corners
I traced the electrics from the switch with the wire disconnected from the solenoid. Steady 12V on numerous tries with the start button. I decided to remove the starter. There is sideways and vertical play in the spline bearing. Not sure if this is normal. The vertical play is much greater than the sideways play. Could the armature be to close to the starter frame at times because of the sideways movement? The starter is a "Mitsuba SM-13".
I traced the electrics and all is fine. Disconnected wire on the solenoid and repeatedly tested button. Constantly over 12V. Removed the starter and have sideways play on the spline of the starter. Also, there is a vertical play. You can shake the starter and the armature moves back and forth. Not sure, but this does not seem normal.
BTW, removing the starter was not a piece of cake. tehe
I had the exact same issue on my 2006 1800 c. I ended up replacing the starter assembly switch because when I cleaned it it would work for about a week or two and then the same thing would happen over. I'm not sure why these bikes are notorious for starter assembly switch issues. I've owned more bikes that I care to remember and this is the only one that has this issue as there are so many threads with the same narrative.