Why does your lumina 31 crank free with plugs out but not when back in?

When you remove the plugs, this removes the compression of the piston in the combustion chamber. There is no force acting on the top of the pistons when the spark plug is out since as the piston comes up, it just pushes the air in the chanmber out the spark plug hole. Once you plug that hole with the spark plug, now there is resistance and as the piston keeps getting closer to the top of the combustion chamber, the pressure increases which is called compression ratio. The higher the compression ratio, the more force it takes to drive the piston to the top of it's stroke. If your starter can't crank the engine with the plugs in it, then I would say you have a weak battery or some poor connections to the starter from the battery. Clean all your connections and fully charge the battery and then see. Removing the plugs just removes the resistance the engine uses to compress the air, add fuel, and ignite with spark to explode and drive the piston down on the crank thus turning it into a circular motion which is then used to spin a flywheel.