Now I am an expert at crank removal (one time) I managed to get inside the case.
Looks like the main drive gear (number18 on schematic above) has worn a flat spot. Hard to see with the camera flash, sorry.
Both my high powered bikes are like this now. It starts as a small skip in power when too much torque is applied at high speed, but over a few hundred kilometers it becomes worse. Pedalling hard makes it go away at the start - I guess it keeps the chainwheel rotating so that the flat spot is moved beyond.
It becomes a bit like wearing out a clutch on car - after awhile you can't get power to the wheel(s) at all.
I loaned the bike to a relative who was constantly putting all the power down from standing start in the highest gear at low speed, with audible loud skipping noise.
You wouldn't take off from the lights in sixth gear in a car but I guess some folks aren't mechanically minded and see ebikes as a 'magical mystical mechanism'.
I can get another large gear from aprilia for about AUD $120, which is still good value when I consider how much petrol I have saved on the work commute. But I cannot see how to get the old one off. The assembly is really tight. Perhaps it is pressed onto the roller bearing (number21) in the centre of it. I don't think it would have a thread internal - nowhere to attach to.
I'll have to remove it destructively I think. Maybe a bearing press...
Unfortunately the small pinion transmitting pwer from the motor to the above gear looks a little worn too. (Top left of second photo). I have a spare from another wrecked bike. Gearbox is full of fine metal, so it'll need a clean out.
Fortunately all the differential gears in the bottom of the case are as brand new.
I was expecting the chainwheel (number 22) to be worn, but it appears ok. At least visually it does.
If I can find out how to get the main gear off and replaced, I think it would be very important to get the gear case bolted as tightly together as possible. The teeth on the motor pinion and main gear need to mesh/overlap as much as possible. There is a rubber seal on the case. I'd recommend anyone who has an upgraded bike to go and at least tighten the hex bolts on the outside of the case as much as possible, you should be able to access these thru the chainwheel by rotating it without pulling the bike apart. You may not be able to tighten them any more than they are - but worth checking I suggest.
If the pinion and gear aren't mating as much as possible, those teeth are going to wear.
Modded Aprilia Enjoy 'Race' & Enjoy 'City -10Y/o brushed Crankdrives.
Globe single spd aluminium with Q100 & 10s bottlebattery.
Dual 1900W BPM/Ezee gear hub for offroad.
900W Ezee Bakfiets / Cargotrike.
"Take the risk of thinking for yourself, much more happiness, truth, beauty, and wisdom will come to you that way." - Hitch.