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Hello my vintage Mustang brothers and sisters, been having an issue, so looking for some thoughts on best resolution path. I have a 1966 coupe 289 (w/ Edelbrock 1404 carb, aside from intake manifold rest is stock enough) paired with a C4 transmission Here’s what I have going on:
car is a bit difficult to start, I have discovered that previous ownership has only a 5/16 line from fuel pump to carb, and is not using the suggested carb connection from Edelbrock
once running, she will sit at idle just fine, but do have some sluggishness when accelerating from stop
shift from 1st to 2nd occurs at 10-15 mph in “normal” acceleration
after that I do not shift from 2nd to 3rd by 50 mph, and once fully warm feels like it’s hunting for a shift that doesn’t occur (I have not tried running it open to 60-70 to see if it forces a shift)
Troubleshooting thus far:
Timing light shows that I might be a couple degrees off, but in okay parameters
I get 15 lbs of pressure at the carb when I vacuum tested it
managed to squeeze my chubby but under it enough to see the vacuum modulator, and found the adjustment screw almost all the way out, I will attach a pic…it’s a bit hard to look at was not able to get a good focus from my phone.
What I believe my issues are: 1) I am not getting enough vacuum, should I adjust to get my 15 lbs to 18? 2) vacuum value may not matter as the modulator is not installed correctly so I am not actually getting a good seal at the modulator, or 3) the modulator is just bad and replacing it is a cheap step and would resolve both 2 and 3… 4) in order to ensure that I get a most vacuum I can (unless is doesn’t matter) I plan to replace the 5/16 fuel line with the right 3/8 fuel line from pump to carb…
It's a shame you can't get an answer for this on this forum. Mine shifts a little early, and I've never messed with it, so I'm not much help. You would get more input on the Facebook group for Mustang restoration: "Mustang restoration, 1964-73 mustangs" You'll have to apply to be a member, but if you have Facebook it is worth it - lots of knowledgeable people there.
Last edited by Mostang66; Oct 7, 2024 at 11:32 AM.
I Would Replace That Modulator. Always a Good First Step. A Plugged Trans Filter Will Cause Issues As Well. I Would Do a Service and Refill With Some Good Type F Fluid and Don't use The Universal Style Fluid. You Might Also Check For Service if The Converter Has a Drain Plug So You Get All of The Fluid Out Vs Just What is in the Pan. Some of The Older Cars Have That. You Will Have To Remove The Inspection Cover and Rotate Engine To Check. I Would Not Sweat The Slightly Low Vacuum But if Timing is Retarded it Will Be Lower Than Normal.
Also Some of The Modulators Have The Screw Sticking Out a Bit and Others Are Recess. See Rock Auto Pics. {Note} First Time I Have Seen That. All The Oem Ones I Dealt With As a Ford Parts Man For 30+ Years Were Recess! 62 Years Old & Still Learning!
Good point about your vacuum from 05stangkc. I have an aftermarket camshaft and only can muster about 14 inches of vacuum. My transmission still shifts early, so I don't think I would be concerned about your vacuum either. My brake booster wasn't supposed to work on engine vacuum after the cam went in, so I added an auxiliary vacuum pump. It never kicks on (except prior to start-up) and the brakes work very well.
The Part Throttle Shift Points Can Also Be Somewhat Changed By The Set Screw & Which Some But Not All Modulators Have. Also Length of Pin. After Installing New Mod Try Some of The Suggestions I Found Here and Note Shift Points as You Change Settings!
NOTE, A Good FACTORY, Not Haynes or Chiltons Shop Manual is Always a Good Thing To Have For These Issues! You Can Get Paper, Usb or Cd Versions! Best Money You Will Ever Spend if Working on Car Long Term.
C4 Issues: won’t shift from 2nd to 3rd
