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As far as how fast the starter spins, I can't find any info for that in my Clymer's manual. Maybe an OEM shop manual does. But, a good guess with my 650, I would say it cranks the engine at about 90 rpm.
If you still have it off, I would take it apart and clean and lube the shafts, and gears in the power end with some never seize.
Also, check the condition of the battery, ie: voltage, acid level and test the acid with a small tester.
Clean the connections when you put them back on. If the bolts that hold the wires on the battery look crappy at all replace them. Clean the connections at the starter well.
Start it up and check for voltage at the battery. It should be from 14.0 to 15.0 volts at 3000 to 4000 RPM.
If not get a manual if you don't already have one and do the tests they explain.
An OEM maual is best if you can find one but a Clymer's is a good second choice.
Good Luck
Ride safe
78 650-C2, Stock engine, Jardine 4-2 Exh., 17-38 sprockets, dyna ignition and coils, coil wiring mod, carb mod.