Have you tried jumping it with your car? Also, what happens if you remove the spark plugs and hit the key?
There was an old 60 HP Evinrude on my 170 when I first got it that had the same symptoms. On an old battery that came out of my Grady (registered 12+ V) it would spin slow and never really get full revolution. I removed the plugs and when I hit the key it shot a bunch of old gas and oil out of the cylinders. I then sprayed a little starting fluid in the cylinders, replaced the plugs, sprayed a little in the carbs and tried again. This time it spun much better, but not as fast as it should. Gave it a little help with my Jeep's charging system (jumper cables), sprayed bit more in the carbs and that did it. She spun over nice and fast and roared to life. :)
