I picked up a tach drive for a small block and installed it Monday. I had a MP vacuum advance dist. in the car and it ran okay but I always admired the tach drive so I found one and swapped it in. Now the engine (Stock stroke 340) wont idle in gear and the in park idle will increase about 200 rpm when it is hot. The engine idles at 1000 rpm in park and 850 in gear when cold. Like I said the car ran fine with the other dist. so I don't think it is a carb issue or a bad coil (I know it sounds like a bad coil). I am going to pick up a cap and rotor for the tach drive because I haven't swapped those yet. Could something inside the tach drive be messed up, though? I pulled a spark plug and it was covered with soot (were a nice tan color with vacuum advance dist).