Very weak spark will not start

The alt. Light came on and blinked then stayed out. I got it home and tried to test the alt. To see if it was bad. It started and I tried a few things shut it off and thought of another thing to try it started ran about 10 sec. And quit. It would not start again. I put the new alt. On and tried it still no go. I pulled a plug and looked at the spark and could just see it. So I got a new coil put it on no change. Put a new distributor on and still very weak spark and will not start. What else can cause a weak spark? Thank you Wayne Ps I had a new engine put in about 5000 miles back