Quote:
Originally Posted by Nickj
Might be the starter motor bushes,
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Have a gold star
I was convinced from the symptoms that it was the starter motor at fault. We tried restarting the bike after a decent run, and although the starter tried to turn it over, it was clearly struggling. I'd believe things like a bad spark plug or sensor if the engine turned over but didn't catch, but in this case although the motor would turn a bit, it couldn't make it past peak compression.
I'd already checked the voltage at the starter on a previous occasion, and that was fine - no issues with the contactor or grounding.
With all that in mind, I changed the starter at the weekend, and today we refilled the oil and tried it.
I'm pleased to report that it started first time with a cheery whirr, which apparently isn't something this particular bike has ever done until now. I'm confident it's fixed (not to mention, somewhat relieved that there wasn't a loud bang and an oily puddle, given that I'm not one of the world's natural mechanics).
I also found the root cause of the starting problem. Once the old motor was out, I took it apart and found one of the four brushes cracked and split. Looks as though it may have been like that since new given the wear pattern, and I'm surprised it ever worked.
Higher temperature = greater resistance = greater losses in the starter, which explains why it didn't have the torque to turn over the engine when hot with a failed brush.
Thanks to everyone for your suggestions, fingers crossed it stays fixed.