MG Car Repair: Ignition coil overheating, ballast resistor, starter solenoid

Question
A few weeks ago, I left the ignition key on in my '76 MGB, burning out the ignition coil, points, and condenser.  I replaced those and the car ran fine in my garage.  I then took it to emissions and failed.  I drove it the 45 minutes to the local LBC shop and they did a better job of adjusting the carb than I had; in 5 minutes it was clean.  I drove it the 10 minutes to the nearest emissions station and it passed.  Less than 5 minutes after leaving the station, the ignition coil overheated and killed.  Now the ignition coil heats up after a few minutes of the car running.  
Did I most likely get the wrong ignition coil from Auto Zone, or is there something else, related or unrelated to the emissions adjustments, that I should be looking at?

Thanks!

Answer
Hi Billy,
To start with the 76 "B" came with a 6 volt coil (as they called it) so if you ran the full 12v to the coil and did not use the resistor wire or used a ballast resistor on the coil it would for sure over heat the coil. If your car had a CAT then your car had electronic ignition which also used the 6v coil. If a pointed distributor was installed in place of the electronic (most did) and someone used a 12v coil without a resistor and you replaced it with a 6v coil then you will have the problems you are experiencing.

Look on the starter solenoid to see if you have two small wire connectors. If so it was meant to have a bypass system that routed a full 12v to a 6v coil ONLY in the start position. When the key is released to Run position the power to the coil comes from the ignition switch and goes through a resistor wire or through a ballast resistor, dropping the voltage down so as not to over heat the 6v coil.

If it is not written on the coil from Auto Zone you can use an ohm meter to test the coil to see which it is. The coil should be 1.43 to 1.58 ohm across the primary and the ballast resistor should be 1.3 to 1.4 ohms.

Howard