Hi XXXXXXXXX,
Have you verified that you only have 12v to the MSD components when the key is in the "ON" position?
There are a few different ways to set up the 6AL. Most likely if it was wrong it would have caused an issue immediately but not always. I think the box is your most likely cause (well what we should check first at the very least) but I need to know how its wired in order to tell you how to test the box itself. 6AL's are great but they can sometimes be moody. This is also why I asked if you only had power with the key on, the box gets a signal from the igntion switch which tells it to "turn on" and then "turns on" the distributor and coil. If the box is getting constant power instead of key on power from the smaller red wire it can cause issues.
Sorry I had to put this through as an answer but the site only allows me to use the question option if I am simply asking a question.
Ok, yes thats good.
The first thing you need to check is that you only have 12v power to the little red wire going to the ignition when the ignition is "ON". If you have power there with the ignition off, that is your problem.
If that checks good then to test the box:
A. Make sure the ignition switch is in the "OFF" position.
B. Remove the coil wire from the distributor cap and set the wire 1/2" from ground, such as the
intake manifold or cylinder head. Do not crank or attempt to start the engine.
C. Disconnect the magnetic pickup wires from the distributor or crank trigger pickup. Turn the
ignition switch to the "ON" position. Do not attempt to crank the engine.
D. With a small jumper wire, short the Green and Violet magnetic pickup wire going to the MSD
together several times.
E. Each time the magnetic pickup wires (Violet and Green) are shorted together or the White wire
is tapped to ground, a spark should jump from the coil wire to ground. If there is no spark
substitute another coil and repeat the test. After repeating the test with another coil, if a spark
still does not occur from the coil wire, remove the small "Red" wire from your MSD going to your
ignition key "On/Off" source and attach it directly to the positive (+) terminal on your battery.
NOTE: Attaching the "Red" wire directly to the positive side of the battery will cause the engine not to
shut off when the ignition key is turned off. If attaching the wire to the positive side of the
battery cures the problem, then you must find a different "On/Off" 12 volt source for the small
"Red" wire from the MSD. If after all of the above tests have been performed and no spark
appears, then your box is in need of repair
If that tests ok, let me know and we can check some other things
Classic Car Mechanic
20 years experience in rebuilding restoring and repairing classic cars