All the above is good advice. Like they said it is most likely something got stuck. So look at everything from the gas paddle forward. Just look for something binding or interfering.
The choke cams are behind the big black round plastic cover, usually on the right side of the carb. Not inside it, between it and the main part of the carb. You will see a cam with several 'steps' and a 'lever' in line with it.
The basic idea of the choke is to open the big flat choke plate in the center AND lower the fast idle as the engine gets warmer. Until it is hot, then the cams fall out of play and the choke plate is vertical and not longer restricting air flow.
There is a heat sensitive spring inside the black cover which exerts pressure to close the plate and let the idle cam raise the RPM. That is why you step on the gas a little BEFORE starting a cold engine, called setting the choke.
As the spring gets hotter it allows the cam to rotate and the lever steps down to a lower spot on the cam. This happens several times until this cam rotates enough to be out of play and the choke plate should be vertical at this point. At this point the HOT IDLE set screw now controls the idle RPM. And this is usually on the right side of the carb where the throttle linkage is.
Spraying some WD-40 on the cam when the engine is cold may help. Also check that all the vacuum hoses are connected. That all vacuum ports on the carb are covered. Check the vacuum line going down to the trans
at the top and bottom. Also the pipe, with several smaller tubes for vacuum lines, behind the carb on the intake. Not all engines have this.
Again - DO NOT change anything like the timing, point gap, etc until you check all the above.
You might ask the local Mustang club for help.
1968, hardtop, 289, C4, seafoam green.
1966, hardtop, 289, original 3 speed, on the road and fun to drive.
1968, hardtop, 200, 3 speed, in work.