Well, the plot thickens...
Yesterday I changed the plug wires, and swapped in a good coil from another car, but when I started the car and starting adding revs, it began sputtering, popping and misfiring at 1500rpm and above (same behavior it's been doing since Saturday night)
So, I took my timing light and first made sure all the new wires were sparking consistently (they were), and then I put it back on #1 and watched my timing scale while I revved it, and WHOA! - I had over 40 degrees of advance at 1500rpm, and over 50 degrees at 2000rpm
So then I plugged the vaccuum advance to watch the centrifugal curve, and as soon as I did that, the car immediately began running "normally" again, and the misfiring completely ceased. The built-in timing curve seemed about right for this engine- it was aggressive, but not insane (it ran from 12 degrees up to 38, all in by about 2500rpm)
Once the car was fully warmed up, I left the vaccum advance corked, and went out for a test drive, and the car ran flawlessly - it ran hard (and fast) well past 6000rpm, and smoothly returned to idle at each stop. No sputtering, no stalling, no misfires. When I got home, neither the coil, nor the dizzie were any hotter to the touch than the intake manifold was (on Saturday night when it died, both of them were WAY hotter than the rest of the engine, they felt more like the header tubes
)
So, the only mystery left is: What's up with the vaccum advance? it was connected to the correct ported source on the carb, and it had never been touched since the day I brought the car home. After my test drive, I pulled the cap and rotor, hooked my hand vacuum pump up to it, and watched it pull and release the advance plate - everything looked normal.
My best theory is that perhaps the vacuum advance
never worked, and something during my drive on Saturday night (some bump in the road, who knows
) finally jarred the advance plate in the distributor loose, allowing it to finally pull the timing forward, and the excessive advance started the car misfiring (which is much harder to hear when you're blasting down the freeway at 3300rpm and 80mph), thereby overheating the coil until it finally gave up (that suspicious plug wire on #3 was probably also a contributing factor, but the new wires should clear that up)
I can't imagine an alternative scenario where the engine just magically started producing more vacuum??? What mechanical condition could cause the exact opposite of a vacuum leak? I haven't messed with the carb, and the plugs say the engine isn't lean - all 8 plug insulators are dark tan (like the color of cardboard) and the electrodes are clean
I'm going to drive it around for a couple more days before I decide that the spark controller is totally off the guilty list, but all indications are that the un-adjustable vaccum diaphragm that MSD used on this dizze is just adding in too much advance... I'll have to decide whether to just leave it capped, or try to find an adjustable one that fits this dizzie.
I also considered hooking it back up, running the rpms up to the point that it misfires, then slowly retarding the dizzie until it smooths back out, but I'm afraid that might leave me with too little initial and centrifugal advance (initial is set at 12 degrees right now) - But I guess it wouldn't hurt to play with it for an afternoon.
If any of you Ford engine gurus have any other insights or ideas, please chime in, but 50+ degrees of spark while cruising just seemed too aggressive to me on 91 octane Arizona winter blend pump gas... Any other opinions on that?