Strange battery issue
 

This issue is for a 2008 Mazda 6 4cyl with a new battery. The car won't start but everything electrical is functioning. I immediately check the positive lead to the battery because of a previous issue of it being loose and not wanting to tighten all the way. This is my girlfriends car and she has not had the positive lead replaced like she should have. I tighten it up even though it doesn't seem too loose. The car still won't start. I loosen it up and I'm getting sparks. Just for ****s, I keep it loosened up and try to start it. It starts up no problem. Ok great, I tighten it again, the car won't start. I try to loosen it up again, again it starts back up. The car only starts when the positive lead is loose and it seems like it needs to be in a certain position to work. I repeat this over and over again with the same results. I figure I'll replace the lead. I do that and still get the same results. Even though the battery is new, I get it tested. The battery is fine. I have a warranty on the battery so they switch it out for a new one anyway. The same results happen. So what the heck is going on!? Why would it receive current just fine when the lead is loose but lose the current when it gets tightened!?

It gets complicated so I won't go into detail but spark gap actually increases amperage output. Check the electrical portion of the ignition switch. Most likely the contacts are carboned up. They are pretty cheap but I can usually take them apart and fix them.