Definitely check and adjust the kickdown linkage. Make sure the vacuum modulator holds vacuum. While the FMX did not come with an adjustable vacuum modulator most replacements were adjustable. Turning the screw in (clockwise) will increase control pressures and make shifts firmer, out will be softer.
Here is the shift speed chart for an FMX with 351C-2V. Although it doesn't have a 3.50 gear listed you can make educated assumptions on what yours should be by looking at the difference between 3.25, 3.00 and 2.75 ratios.
View attachment 63001
You can look up the tire diameters and calculate your tire diameter with the spreadsheet in this thread. The original tire sizes are in the "Original Sizes", others are in the "Tire Data" tabs
The attached spreadsheet is set up so that anyone can enter a tire size and get the approximate diameter, widths, and recommended rim width. Axle ratios can also be entered and the engine RPM at 60mph is shown, based on the tire size. The engine RPM can also be entered to check speeds, based on...
7173mustangs.com
Definitely check and adjust the kickdown linkage. Make sure the vacuum modulator holds vacuum. While the FMX did not come with an adjustable vacuum modulator most replacements were adjustable. Turning the screw in (clockwise) will increase control pressures and make shifts firmer, out will be softer.
Here is the shift speed chart for an FMX with 351C-2V. Although it doesn't have a 3.50 gear listed you can make educated assumptions on what yours should be by looking at the difference between 3.25, 3.00 and 2.75 ratios.
View attachment 63001
You can look up the tire diameters and calculate your tire diameter with the spreadsheet in this thread. The original tire sizes are in the "Original Sizes", others are in the "Tire Data" tabs
The attached spreadsheet is set up so that anyone can enter a tire size and get the approximate diameter, widths, and recommended rim width. Axle ratios can also be entered and the engine RPM at 60mph is shown, based on the tire size. The engine RPM can also be entered to check speeds, based on...
7173mustangs.com
This (get and adjustable Modulator and adjust it) is what I was going to suggest, before dinking with the governor. The aftermarket (adjustable) Vacuum Modulator may be able to provide the degree of adjustment needed to get you back on track.
As for the AOD swap, we did that in out 73 Mach 1 last year. Our Mach 1 has 3.5:1 TractionLok gears in it, which are great for decent around town spirited driving performance. And at 60 MPH the engine RPM was not all that bad, about 2,900 RPM or so. But, with the AOD in 4th/OverDrive at 60 MPH the engine RPM is now just about 1,900 RPM - which makes for a far nicer cruising experience. You are going to LOVE it.
But, the AOD needs to be set up correctly with the Throttle Valve (TV) pressure at the carburetor. This is likely something new to you, unless you have worked with AODs in the past. Otherwise it may/will sound spooky, and perhaps it should because if you do not set the TV pressure high enough you can smoke your clutches. I can assure you, this is not a difficult thing to do, and in our case instead keeping the initial setting of the TV pressure to 35 PSI I reset it to 38 PSI. And I am about to re-reset it again, this time to 41 PSI. Why? Well, I am glad you asked...
First of all, setting the TV pressure higher than factory spec at 35 PSI is not going to hurt anything. I am doing it to move the light throttle shift speeds up a little, as the upshifts are occurring a little sooner than I like. Also, with the higher TV pressure setting the upshifts will be a little firmer - which I also want. I doubt I should need to go higher than 41 PSI, but if I do it is not going to hurt anything.
I have attached a few files showing the parts and vendor lists I used, and another file with a bit of background on our swap experience. In the document with a sketch of our experience I have a bunch of YouTube videos I tool of our swap project. I really think you will find it useful. Included in the files and YouTube videos is a section on where we got the pigtail for the AOD Neutral Safety/BackupLamp Switch (NSS), as it is nothing at all like the NS/BU Switch on the C4, FMX, and C6 transmissions. In one of the videos I show where I spliced the AOD NS/BU Switch into the old C4 NSS harness (not into the automotive harness, I left that along). It worked perfectly.
Another aside re: the NSS... If you have a 1973 Mustang the original NSS has 6 wires. Two are for the Neutral Switch, two are for the Backup Lamp Switch, the the last 2 are for the likely no longer in use Seatbelt Interlock circuit that was no doubt disabled (legally) as the Interlock circuits proved to be a debacle and drivers across the country revolted - and the federal government allowed them to be disabled. I never see 1973 cars with them working, and the AD pigtail will not have wires to connect to the InterLock system anyway. Nothing to worry about. Just follow the direction on the one YouTube video I have the link for in one of the attached document files and you will be fine. I did solder my pigtail into the NSS harness, and used shrink wrap tubing to protect it from moisture. I do not drive the Mach 1 unless the road are dry, but every once n a while there is standing water just waiting to splash the car. I knew if I used simple crimp connectors the day would come where the NS/BU Switch leads may end of having a problem.
If you run into any questions or concerns re: your AOD project please feel free to email me (or post in this forum as I have been scouring the new entries for the past few months and plan to continue to do so for as long as I can). My email is
[email protected].
