Check all the cables for corrosion and make sure they are cleaned and TIGHT. It takes a lot of amperage to push that starter to get the engine to turn over, and if the connections are loose or dirty then you don't get 100% current flow and under a load it'll do nothing. Actually, the cables will probably heat up too. Make sure all grounds are good too.
Also, I wouldn't try starting or running an engine with loose spark plugs. The power of compression on a loose plug will cause it to shoot out like a bullet! When this happens say good bye to the plug threads (head side) and perhaps an eye or anything hanging on the wall near by. My relatives once put a hole threw an outboard motor housing because they cross threaded the plug. Neighbour did the same with his car and the hood.
Cheers!
--FWD