1960 GMC 305A v6 hard to start

[Lineman]

I am confused. Lets say everything is good, working fine. Engine cranks at 100rpm on the starter with the battery at full charge. If you hook another battery to it, it should still crank at just about 100rpm shouldn't it?

So when the Pirate hooks the second battery to the motor and it starts right up sounds like he narrowed down the problem? Battery, connection, is the ground cable connected to the engine and not the frame? Things like that. Put a meter on the battery when he cranks it over could be the starter pulling too many amps you can check that with an amp meter while cranking but look at the voltage at the same time because if the voltage goes way down the amperage is going to go way up. If the voltage goes way down you found your connection issue. Switch out the battery, put on good cables, not those stupid bolt on terminals etc... Just stuff to try....

When my truck sits for over two days it cranks and cranks so I put an electric fuel pump I use just to prime the carb and then motor runs of the mechanical pump. Not your issue yet. Let me know what you think.

Jay