You added a wire to provide battery voltage while cranking the engine but now there is no power in the run position. You need to bypass the resistor wire as it is not needed for this ignition system and repair the ignition power wire to the coil.
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Hard to tell from your picture, but it looks like you put the new red wire on the wrong small terminal at the starter relay, should be at the one that had nothing attached to it.
Also did you leave the other wire that was at the coil + terminal hooked up?
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lol ā¦ you seem to be on the right path anyway. The starter relay on my truck has one terminal labeled āsā which is an input to the relay. It is 12 volts when the key is in start, 0 volts otherwise. When 12 volts the relayās internal switch connects the battery to the starter motor, cranking the engine. The relay has another terminal labeled āiā , which is an output. Inside the relays thereās a switch that connects the battery directly to the coil positive terminal when āsā is 12 volts. So when āsā is 12 volts, the engine cranks and the coil has full power to make a bright spark. When the key returns to āonā (from āstartā), the switch inside the relay disconnects the battery voltage to the coil (and to the starter). To keep the coil powered while engine continues to run, thereās another wire from the ignition switch to the coil positive that starts as 12 volts at the ignition switch, but it is a resistor wire, so by the time it reaches the coil it measures about 8 volts, which is enough to keep the engine running, but not enough to start it.
Seems like the OP has left the buildingā¦Nope, still here!
I guess those old cars werenāt as simple as he thought they wereā¦
OP posted 30 minutes ago, probably has another appointment now. I guessing OP is starting to enjoy the experience. Newspaper puzzles arenāt easy to solve but I still enjoy trying to solve them. Hereās one that I found quite difficult
Two rectangles, one is 5 X h, the other is W X H, and larger, has both twice the area and twice the perimeter. What are the dimensions of both rectangles? All dimensions are exact integers.
The coil+ terminal does not need to be run through the starter relay at all, it needs to be wired into a 12V keyed run positionā¦ the resistor wire needs to be 100% eliminated from the entire ignition systemā¦
If you canāt figure that out, you can always run the coil+ wire to a toggle switch and mount it somewhere under the dash or in the dashā¦ Temporary if needed until you can get this all figured outā¦ Just make sure the OEM coil+ wire is not able to short to ground anywayā¦
You can add a seperate 4 or 5 terminal relay (link posted in prior link) if neededā¦
BTW, The corroded wire (that you posted a picture of) requires replacingā¦
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The āIā terminal on the starter relay/solenoid is only energized with 12 volts while cranking., it has nothing to do with the engine running.
Itās only used to supply the full 12 volts to the coil while the key is in the āCRANKā position so it will start, once the key is released to the āRUNā position this wire is no longer being used.
His car already runs, heās just not getting voltage while cranking.
I know how it worksā¦
That is why it is NOT needed, if the resister wire is not used then that means that the I terminal is NOT needed as the coil will have 12V all the time period not just when crankingā¦ again, any and every time the key is in the RUN position, the coil+ terminal HAS to have 12Vā¦ SO I will say it again, the starter relay does not in anyway need to power the ignition coil at anytime while running the MSD ignition system on the OPās mustangā¦
This is a modified vehicle/ignition system, NOT STOCK and the standard wiring NO longer appliesā¦ lol
This is a VERY simple 3 wire distributer on a very simple engine/carā¦ Stop over thinking itā¦ 
In the last picture it looks like the OP wired the coil to the S terminal on the starter relay, that is why it now starts but dies as soon as he lets off the keyā¦ AGAIN No coil wires should be run to/from the starter relay in anyway shape for or fashion on this MSD systemā¦
Whether that wire is needed on the āIā terminal or not depends on the internal configuration of the ignition switch. One way or another, 12V must be supplied to the + terminal of the coil both during the start cycle and the run cycle.
Anyone have access to an old Motors manual or Glennās motor manual from the late 60ās? They used to have wiring diagrams in them.
Edit: I just looked up the ignition diagram from a 65 Ford/Mercury shop manual on the oldcarmanualproject.com site and there is no internal connection in the ignition switch to provide power to the coil in the start position. It gets power from the āIā terminal during start.
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I understand itās been modified, but on a 60ās Ford, there is only 1 wire that supplies voltage to the ignition system when the key is in the āCRANKā position, that wire is from the āIā terminal on the starter relay/solenoid.
If this system was modified correctly, the OP wouldnāt be here asking for help.
I also have a suspicion he removed the wire that was connected to the coil +, as to why it now starts but wonāt stay running, it needs both of those wires on the coil + to start and run.
Before, wonāt start, runs good. Now, starts good, wonāt run. With classic cars, seems itās always something ā¦ lol ā¦ Iām guessing OP almost has the ignition part of the problem solved, but has some more work ahead to solve the accelerator pump problem.
Yeah I didnāt realize that (or forgot it lol)ā¦
Well def needs the one on the I for cranking (as yāall said), but only needs key on 12V while runningā¦
The OP technically can run one wire (inline fuse maybe) from the battery to the coil+ and bypass everything else including the big relayā¦ Just to see if the thing will start and run normallyā¦
So if the OP would just hook the one new wire he hooked to the Relay S terminal in the picture to the battery+ (able to remove fast to turn car off) and see how it starts and runsā¦ Once we knows that it works like that, then he can wire it up properlyā¦
Very trueā¦
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Yep, thatās if he comes back.
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One of Fords better ideas, thereās easier ways to do it.
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Got it started on Friday. 3 cranks. Went for an hour long cruise.
Spent 6hrs trying to get it started yesterday without success.
Got it started this morning on 3 cranks again. Drove it directly to the garage it is being stored at for the winter.
Thanks for the support.
Try it again in April
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Well thanks for telling us what you did different???..??..
Would be nice to know since a lot of us are putting in a lot of time on thisā¦ lol
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I said Iād be back in April. So here I am to.share the cause of the problem. It was the electric choke. It was loading down the 12V to the ignition. I removed the 12V from the choke and the car starts right up on 1 crank. This has been repeated 3 days in a row.
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Congrats on finding the trouble-maker there Quig! My truckās electric choke ā the electric part anyway ā hasnāt worked in 20 years ā¦ lol ā¦ but as long as driven at least once every week, it still starts right up, idles, & runs well.
As I understand how those electric chokes work, even though they are powered all the time the ignition is āonā, they have an internal switch that turns the heating coils off after they reach operating temperature. So their only function is for the first few minutes after a cold start, presumably to lower HC emissions. After the warm-up time, they no longer do anything.
With no electric choke good idea to make sure the thermo-siphon tubes are connecting & heating the choke assāy. I had to install a new tube-connector on my truck b/c the old one fell off the exhaust manifold. Visually follow the choke-heating tubes to make sure. There should be one from the air cleaner (or carb) to the exhaust manifold, and another (usually metal) from the exhaust manifold back to the choke assāy. If your choke plate is completely closed w/a cold engine, engine off. Then it pops slightly open as soon as you start the engine. Then it gradually opens all the way as the engine warms up over the course of 3-4 minutes, you should be good to go.
Iād have never guessed your symptoms could be caused by a presumably shorted-out electric choke. That was a tough one.
One question remains though: On my Ford truck I think the electric choke is powered directly from an alternator winding, not the 12 v from the battery. Done that way presumably so the electric choke is only powered when the engine is running. Is your Mustangās different?
My old chev had the vac pull off line plugged for my choke by previous owner. Never ran good upon starting till I figured it out. Had to dig into it.
I guess the configuration varies engine to engine. Thereās no choke-related vacuum hose for my Ford truck. The choke assāy is bolted to the carb using a gasket, and the vacuum path which pulls the heated air into the chokeās coil-spring thermometer gadget is internal, passes through a hole in that gasket. Youād think that hole would clog up w/time and cause the choke to stop working, but it never has.
Or maybe the vacuum hose you had a problem with was for the choke pull-down? Thereās no vacuum hose for that function on my Ford either.