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Discussion Starter #1
Hello all, I am new here. I’m 16 so I don’t have a lot of experience nor tools. I got a 66 mustang for my 16th and have run into some issues. It’s a modified 289 with a c4 auto transmission and a Holley Performance 2 barrel carb. It wasn’t idling last Tuesday and I managed to get that fixed but it started misfiring badly with backfire too! It has never done that. We had a friend come look at it and said to check the timing. Unfortunately I don’t have the knowledge to do so so we were going to take it to a shop. When I put it in reverse on Thursday, it started accelerating on its own. At first I thought it was the throttle that had gotten stuck but it only seems to do it in reverse. I’ve talked to my family which is made up of people who love cars and no one seems to know what is going on. Looking for some help. Thank you.
 

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Welcome! I'd suggest taking the opertunity to learn some basic skills working on older cars. They're pretty simple. Could very well be that your timing is off. It could also mean a lean fuel mixture. A timing light is very easy to use and not expensive. Invest in a Ford shop manual. It'll explain how to do a lot of procedures.
 

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Discussion Starter #3
I do have a shop manual that my dad got me. Unfortunately this is my main car so I don’t have a whole lot of time to work on it. The timing can be fixed fairly easily with the help of some of my family. We are more stuck on the unexplained self acceleration. We are afraid to drive it anywhere and can’t can’t get a tow truck close enough to the garage.
 

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First of all, congrats on getting such a cool car for your 16th birthday! You'll have years of fun with it!
Second, where are you located. If you're close by, one of us may be willing to come by with a timing light and some experience to see if we can help you diagnose your problem.

Following is what I'd do next...
  1. Check to make sure the throttle linkage moves smoothly throughout it's range of motion, and is not catching or hanging up on anything. Do this with the engine off, just moving it through it's full range. A can of carb and choke cleaner may come in handy here if the linkage is dirty and gummed up.
  2. Add a double throttle return spring if it doesn't already have one. https://www.amazon.com/Spectre-Performance-Throttle-Return-Spring/dp/B00062ZD5E/ref=pd_sbs_263_t_0/146-3993860-8214311
  3. Verify your vacuum lines are good, or better yet, just replace them all to be sure you're getting a good vacuum signal to the distributor and the C4.
  4. Check your timing - there are a ton of inexpensive timing lights ($30 or less) available from just about anywhere - O'Reilly's, Autozone, Amazon, Walmart, etc.
  5. Replace the vacuum modulator valve on your C4.
That's a start. Let us know if the problem persists after the above.
 

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Welcome to the forum!
If you edit your profile to include your location (city & state), you may find that there is a member not far from you that would help. I would have no problem mentoring a young enthusiast and allowing them to use tools that sit in my toolbox collecting dust. If you are in my area, contact me.
 
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It won't help your idling but for the unintended acceleration take a look at your motor mounts.

If they go bad it's possible for the engine to rotate a little and when it does it can bind up the throttle linkage making it rev up more.

If you had the hood open and turned the car on and put it in reverse somebody watching it would be able to see it happen pretty easily if that's the issue.
 

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Discussion Starter #9
We did check the mounts. Even had someone watch it with the good open. Nothing wrong with them.
 
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