This sounds like a cardale drum type spring,central in door frame aprrox 8"
in dia

SORRY FOR SHOUTING HERE BUT UNDER NO CIRCUMSTANCES SHOULD A UNQUALIFIED
PERSON ADJUST THIS MECHANISM,EXTREME DANGER TO YOUR EYES AND LIMBS WILL
OCCURR AS HAS HAPPENED TO OTHERS WITH THIS DEVICE.

It is not designed to take extra weight to the door and I cannot stress
enough the above advice,even professionals in the trade will shy away from
these.Cardale no longer make these devices as the machine that wound them
broke down and was uneconomical to repair.

You could assist the door by using a pulley and weight system/