Small Engines (Lawn Mowers, etc.): Craftsman riding mower wont turn over

Question
I have 2 mowers that don't spin fast enough to start. I have a charger hooked up to the battery trying to start them. Both engines are Briggs an straton, one 15 horse think the other is 13. Both are identical. When I turn the key the engine try's to spin and does so maybe once all around but then stops again as if something is holding it back. A friend of mine thought I should try another starter so I took it all and tried it on the ground and it worked good. I have taken carburetor off and clean it up drained the tank and line put new fuel in it check to see if I had a good spark (it does), have taken a wire brush to flywheel and coil thing reassembled adjusted gap same thing. With spark plug out engine will turn over like it should but when you put plug back in it will not turn over again. Have tried another starter from another mower of mine that should be good, got the same thing. I have had this issue with numerous mowers in the past and would really like to know what is the problem.

Answer
Do a Google search for "how to measure voltage drop".

Also, make sure you dis-connect the battery cables and clean the battery terminals and cable ends with a wirebrush or sandpaper...I have fixed a lot of starter problems just by cleaning the connections even though no corrosion was visible.

FYI, I do not find very many bad cables but I have had a few.  

Let me know if the cables test good when performing a voltage drop reading across them.

Eric