Small Engines (Lawn Mowers, etc.): Riding lawnmower will not start, specific gravity test, battery cells

Question
I have an 18HP twin cylinder B&S engine on the mower.  The engine barely turns over and will not start but there is no unusual load on the engine; it still turns just as easy by hand as it always has.  It almost seems like it isn't getting juice but the battery and solenoid are fine.  I tried my car battery (but my car was running so I guess you could say I tried my car alternator).  I have even bypassed the solenoid  with a direct connection to my car battery to the starter and it seemed to act the same way.  I took the starter apart and inspected/cleaned the brushes etc. so they are fine but this didn't help.  What else can go bad in the starter?  Is the starter most likely the problem or could it be something else.  For some time the mower would be hard to start but a little turn by hand before another try usually got it going.  

Answer
It could be a battery battery cable, connection or ground connection.  I have fixed 99% of all starting problems by removeing and cleaning the battery connetcions; cable ends, ground connections and battery terminals.

The starter armature could be bad.  You can use a DC shunt and measure the current draw and starter RPM or you can use a growler to check the windings.  Most people do not have these tools but I rarely use them to troubleshoot and fix starting problems.

Man, taking that starter apart is a real pain.  Have you done a specific gravity test on the battery cells or had it load tested?
Will the starter spin the engine rapidly with the spark plugs removed?
Let me know.
Eric