weird ignition issue after installing tach

joe- the tach has a metal body, so I'm guessing the screw is grounding the system to make the tach work (it's an old hole- the tach was mounted there years ago, but I just put it back in)- I started wondering myself if maybe the screw hit a wire, but it's only about a quarter-inch long machine screw- still, it's possible I guess.
67Dart273- yep, I hooked the tach to the neg. side of the coil. Everything is working fine as long as the tach's ground wire is disconnected, so I'm pretty sure everything else is still connected normally (it still starts and runs fine as along as that ground wire is disconnected, oddly enough...

thanks guys.