Try again with a car battery via jump start cables connected.
If that shows the same result, most likely the coals/sliding contacts in the starter motor are worn. Check the springs as well. Repair kits available, at least in the USA.
There shouldn’t be a difference when using the starter motor, either with cover fitted or not.
Is there a paper gasket used or just “fluid” sealing compound ?
There are two reasons for starter malfunction:
– One is the starter itself (no power)
– The other reason is the starter freewheel clutch.
If the starter has no power, it turns the engine or not, just stops working at some point or works very slowly.
The freewheel clutch is a very common issue. It does not engage to turn the crankshaft, but the starter turns up to this freewheel clutch. That makes a terrible noise.
What is it in your case ?
Cheers, Michael (from Germany)