Originally Posted by
marc69
There is more pressure in an engine to turn over with the plugs in, if your starter is gooosed and shorting itself out, it will struggle with the pressure of turning with plugs in and as mowgli said, the cables will heat and in this case overheat.
This is my totally unsafe and obviously unrecommended methid for starter testing. Remove the starter and use your battery and a set of jump leads. Thick gloves and put the earth lead and current lead to the appropriiate places, it should spin fast and freely.