Mixer going too fast despite adjustment

Hi folks!

Got a bit of a weird one and I was wondering if anyone had seen something similar.

It's an American model mixer, KSM150 (KSM150PSOB), came to me initially with uncertain history and no speed control. (I'm in the UK, running it off of a step-down transformer, which I know isn't ideal, but I have anecdotal evidence that this works fine for many friends with US model mixers).

When it turned up, the weights were disassembled from the governor and they were loose in the case.

I reassembled the governor and replaced the phase control board (making sure it was the correct part for the US model), and after that there was a difference in the speed settings, but all the speeds still seemed too fast no matter how I adjusted the speed control plate.

I then replaced the governor AND the speed control plate. The changes in speed between settings seem more pronounced now, but stir is still more than twice as fast as it should be no matter where I set the speed adjustment screws.

I might just have to give up on it, but thought I'd ask in case anyone knows what else it could be?

Thanks in advance!