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On the solenoid, there should be two small posts horizontally from each other...do you have the starter wire hooked up correctly? Looking at it from the back, there should be a pink wire going to the post with the 'S'.
You could have high resistance in one or both cables. It would be hard to check resistance without a load on it. If either cable looks suspicious, corroded or you have "repair" ends on them then replace. Make sure ground cable is attached to block or intake manifold boss. Do not attach it to an exhaust component or piggy back it to another wire.
Post a pic of the starter clearly showing its connected cabling.
black is ground wire to battery