Can a starter motor get weak

[Mostin3]

Hi guys, as previous posts suggest I?ve been trying to fix the starter system on my bike for ages but think I?ve got a few faults leading me up the garden path. I replaced the bendix the other day with a new gasgas item and it?s now engaging every time but as soon as it engages, the motor stops spinning and the motor gets very hot. This would suggest a bad battery, but the battery is a new lithium ion and is giving me a reading of 14.4v. I have also hooked it straight up to my car battery with jump leads and getting exactly the same symptom, ruling out any of the bikes cabling.

It total, I have replaced, the bendix, the two shafts, both helicoidal gears, all New bearings and battery. The only thing left to replace is the motor.