They way I read it you're cranking (or kicking) it over with the decomp lever pulled in?
If you are, Don't.
It's only for kicking it over to TDC before kick starting it. Which you dont need to do anyway, imho. Or you can hold it in whilst cranking over to clear a bit of water out if you've drowned it.
Maybe I read your post wrong, but my 2c worth.
If its cranking really slowly, chech started leads, battery and starter solenoid earth, and/or consider removing and rebuilding starter. Could be brushes are seized in place and not contacting with commutator, or there's am issue in the windings.
2007 640 Adv
2000 380 Exc