On a 12V battery its NOT the cables... really. Unless you are using an extension cord not anything resembling an automotive battery cable. For a 6V battery, yes. 12V, no.
Take out the spark plugs.
If you hit the foot pedal and the motor does not turn over check TWO things: is the contact switch in the starter working (do you have a polarity reverse switch? Is it wired correctly?) and is the starter engaging (can you hear it turn a bit as the bendix engages the ring gear).
Of all the things to go bad, the contact switch on the starter goes first (copper eroded by arc).
Even with a POOR GROUND a starter should have the mechanical advantage to turn over a motor even slowly with no plugs. Remember were at 12 Volts.... grounds play a much smaller role.
If the starter cannot turn over the motor, then clean the starters commutator (you need special stones for this, cost about $30 for a box). Here is how you use them:
http://www.tm9-801.com/forums/viewtopic.php?f=3&t=739&hilit=commutatorYou motor *might* be tight... did you plastigauge the bearings??
