Engine 85 GT Backfires through Carb

DienstXIV · Oct 3, 2025

Hi Everyone,

I have a 1985 Mustang GT 5 speed. I have been having this issue where when I go to start it, it will fire and then die immediately. The tach will go to zero but the engine will still be spinning and if I hit the gas I get flames and or smoke shoot through my air cleaner. Please see video, unfortunately I didn't get the back fires on the video, just how it is hard to start.

I have a 600 CFM Holley Street warrior I just put in and before I had a 670cfm Street Avenger that had a vacuum leak that did the same thing. I thought the vacuum leak was the culprit but I am not so sure now.

Here are some of the things I have already checked and replaced
1.) checked ignition timing, the car is at 12 degrees BTC at about 800rpm
2.) changed sparkplugs with 0.045 gap
3.) tuned the airflow ratio for Max vacuum at idle
4.) replaced the ignition coil with and MSD one
5.) changed distributor rotor and cap

Tomorrow morning I will check for vacuum leaks with my smoke machine but other than that anyone have any idea what it could be?

One last thing. My fuel return has been removed and plugged at the mechanical fuel pump and I have a gauge on the fuel line. I am running 10 psi going to the carb. The Holley manual calls for 5 to 7 psi. I have had this car since 2012 and the fuel return has always been plugged and this issue just popped up this year.

Thanks in advance and cheers,
Alex

Blackhawkxx · Oct 3, 2025

10 psi is way too much for a carb. Backfiring through the carb is often a timing issue, yes I did see that you checked the timing, did you do it with the vacuum advance hose removed and plugged?

DienstXIV · Oct 3, 2025

Blackhawkxx said:
10 psi is way too much for a carb. Backfiring through the carb is often a timing issue, yes I did see that you checked the timing, did you do it with the vacuum advance hose removed and plugged?

Yes, I plugged the vacuum line, I checked the timing with it removed and plugged and with it plugged in and there was no difference. The vacuum advance is plugged into the vacuum advance port on the carb and there is almost no vacuum at idle. I checked it at 2000rpm and the vacuum advance is working. Could it be a bad distributor? It only happens at start-up when you can't really check the timing.

General karthief · Oct 3, 2025

First, I've never seen a mechanical fuel pump with a return line but hey, I ain't seen it all, yet.
Second pull and plug the vac advance hose and see if the carb acts better.
Third, check that the carb base is compatible with the opening in the intake manifold, some carb types need a spacer between the carb and a stock manifold.
What manifold are you running?
Oh, and don't throw that stock coil away, the MSDs are notorious for failure.

DienstXIV · Oct 3, 2025

General karthief said:
First, I've never seen a mechanical fuel pump with a return line but hey, I ain't seen it all, yet.
Second pull and plug the vac advance hose and see if the carb acts better.
Third, check that the carb base is compatible with the opening in the intake manifold, some carb types need a spacer between the carb and a stock manifold.
What manifold are you running?
Oh, and don't throw that stock coil away, the MSDs are notorious for failure.

Good point, I will try starting the car with the vacuum advance plugged and see what happens.

The intake manifold I have is an Edlebrock 2121 and it seems to fit the

Is there anyway to check the ignition coil?

General karthief · Oct 3, 2025

Remember there is manifold vacuum and (I believe it's called) port vacuum,
Manifold vacuum and port vacuum are opposite operations, manifold vacuum exist at idle, port exists above idle, like when accelerating, swap the vacuum line from the barb under the throttle blades on the base plate and find the barb above the throttle blades, if it seems better recheck the timing,
That intake is compatible to a square carb, all 4 throttle blades are the same size.
The coil either works or not, for the most part, the MSD stuff is outsourced so quality control is suspect, lots of failures, honestly the stock Ford coil is pretty resilient, my original 150k mile coil is still working after I swapped it into a 5.0 F150......
Explain the history on this car, when the issue started and what has been done to fix it this far.

AeroCoupe · Oct 4, 2025

Pretty good read on this here:

Why does my engine still try to run after I shut it off?

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Blackhawkxx · Oct 4, 2025

Are you sure your plug wires are in the correct firing order for your roller cam engine? They use the 351 firing order, different from the old 302 flat tappet cam order. That is all assuming it is the factory engine with a roller cam.

DienstXIV · Oct 4, 2025

Blackhawkxx said:
Are you sure your plug wires are in the correct firing order for your roller cam engine? They use the 351 firing order, different from the old 302 flat tappet cam order. That is all assuming it is the factory engine with a roller cam.

I can double check the wires but this issue came up before I touched the ignition system. The car is a roller cam, when I changed the intake I noticed the lifters had rollers.

DienstXIV · Oct 5, 2025

Blackhawkxx said:
10 psi is way too much for a carb. Backfiring through the carb is often a timing issue, yes I did see that you checked the timing, did you do it with the vacuum advance hose removed and plugged?

So I removed the vacuum tube for the advance and plugged it at the carb and the distributor and the car fired right up, no problem! It ran kind of rough until I reconnected the vacuum line. Tried to start it again with the vacuum line connected and the issue came back

Could this be a sign there is something wrong with the distributor? Maybe at start-up for some reason it advances way too much? Maybe I just need a new distributor?

Blackhawkxx · Oct 5, 2025

DienstXIV said:
Could this be a sign there is something wrong with the distributor?

More likely the vacuum advance unit is the problem.

DienstXIV · Oct 5, 2025

Blackhawkxx said:
More likely the vacuum advance unit is the problem.

Are you talking about the vacuum advance unit on the distributor or something else? I don't think you can just replace the vacuum advance unit on the distributor

85GTStangGuy · Oct 5, 2025

I think you can...

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Blackhawkxx · Oct 5, 2025

DienstXIV said:
Are you talking about the vacuum advance unit on the distributor or something else? I don't think you can just replace the vacuum advance unit on the distributor

Yes on the distributor and yes, they are easily replaced.

https://shop.advanceautoparts.com/p/carquest-premium-distributor-vacuum-advance-vbc1041/12549945-P?selectedStore=1204&searchTerm=vacuum+advance+unit

limp · Oct 6, 2025

Pull the vacuum line off the carb and install it into a hand held vacuum pump.. give it a couple of squeezes and watch if your vacuum advance moves.. then watch to see if it holds vacuum....

DienstXIV · Oct 6, 2025

limp said:
Pull the vacuum line off the carb and install it into a hand held vacuum pump.. give it a couple of squeezes and watch if your vacuum advance moves.. then watch to see if it holds vacuum...

I'll get one of those vacuum pumps at Harbor Freight tonight. You got any idea what the 2 or 3 wires going into the side of the distributor do? I really think it's more of an electrical issue with something and it's something that's happening that I don't know about. I unplugged the vacuum line and revved the engine up to 2000rpm and then did the same thing with the vacuum line plugged in and the advance was much more with the vacuum line plugged in.

limp · Oct 6, 2025

DienstXIV said:
I'll get one of those vacuum pumps at Harbor Freight tonight. You got any idea what the 2 or 3 wires going into the side of the distributor do? I really think it's more of an electrical issue with something and it's something that's happening that I don't know about. I unplugged the vacuum line and revved the engine up to 2000rpm and then did the same thing with the vacuum line plugged in and the advance was much more with the vacuum line plugged in.

You said you thought it might be the vacuum advance. I gave you a way to diagnose it...

AeroCoupe · Oct 7, 2025

DienstXIV said:
I'll get one of those vacuum pumps at Harbor Freight tonight. You got any idea what the 2 or 3 wires going into the side of the distributor do? I really think it's more of an electrical issue with something and it's something that's happening that I don't know about. I unplugged the vacuum line and revved the engine up to 2000rpm and then did the same thing with the vacuum line plugged in and the advance was much more with the vacuum line plugged in.

The “two or three wires” are for the magnetic pickup. They plug into the Ford Duraspark ignition box or an aftermarket ignition box like this:

DienstXIV · Oct 7, 2025

Thanks! I decided to just buy a new distributor and the duraspark module, I will keep you guys posted

85GTStangGuy · Oct 7, 2025

Don't write off your oe distributor.... it may still be better than a replacement one.

Engine 85 GT Backfires through Carb

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wonder how much it would cost to ship you a pair

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wonder how much it would cost to ship you a pair

lube between the nut and the face. I know my lubes

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wrap a little cheese around it and its a done

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wrap a little cheese around it and its a done

lube between the nut and the face. I know my lubes

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