Question for my son.. He has a 74 Camaro and sometimes his car starts like it had a dead battery.. barely cranks.. He got a new battery and it does the same thing.. Fully charged... If we just start it from our running truck, it starts good.. What would be drawing the power from the starter? Thanks in advance...

 

Probably a starter on it's way out would be my guess.
Check the cables and terminals to be sure they are shiny clean and tight on both ends.
Does the car have headers? Heat soak is a common problem with headers,they start good cold and barley crank when hot.

 

I would also check all your wiring .You might have a short that is drawing down the power .If possible to give more information .So well can help you.

 

Battery cable is one thing that get missed a lot. They should be replaced every 15-20 years. They will build resistance over time. When you start a car from a new fully charged battery you are getting 12.6 volts but a running car can be as much as 14 volts. So that means either the starter needs the extra power or the battery cables are stealing some of the power.

Cleaning the connections on both ends of the ground cable and the main power cable may help. It is very common for ground cables to build up resistance where they meet the engine. If you would like to test this search youtube for voltage drop test.