85 carb’ed mustang keeps dying

Alextheornery

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Jan 13, 2021
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Hey guys I’m new here, and it’s a long post so I’m sorry but I appreciate input. so I’ve been restoring a 85 mustang w/ a 302 small block, and a Holley 4barrel. It was sitting for many years by the time I had inherited it and it sat a couple years in my possession. Before it had sat, it had the motor and tranny replaced brand new. So in my possession it at first wouldn’t start so I replaced fuel pump. Started right up no issues. Since then I’ve gone through and replaced many things (starting coil, alternator, things like such) Couple weeks ago it started having an issue where I’d go for a drive and when it got to normal op temp it would die and I’d have to wait 10 min and it pups start back up. Then it got worse with me be able to go shorter and shorter distances before it stalling. One day I went for a drive, made it about 3/4 of a mile from the house and it died and refused to start. Replaced all the electrical under the hood and it still refused to start. It would want to crank over if we sprayed into the carb but it wouldn’t stay running. Determined it was fuel delivery problem at that point. So from there, dropped the tank and it was actually immaculately clean but still blew through the lines with an air compressor. Hooked everything back up and it started right up and ran fine on a test drive. That was yesterday. Today I took it for a short drive and I made it about 2-3 miles before it acted up again. Wanting to stall when coming to a stop, losing power when hitting the gas, stalling at my destination and refusing to start. About 2 hours later I was able to crank it over but it was not happy. Really bad idle (about 250 rpms lower than it should) as well as backfiring when reving the engine past 4K. I’m absolutely baffled and don’t want to replace the carb yet if it’s not the issue. It has its own minor problems such as a small hesitation but the stalling and acting like no fuels going into it is new. Float levels are adjusted properly and I got a glass filter and can see gas moving where it should be. Thank you guys
 
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Noobz347

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What tests have you performed while the vehicle is in the no-start condition?

Are you getting spark and fuel?
How old is the fuel?
What does the filter look like?

What do your plugs look like?
What's your coil input voltage while cranking?
 

Alextheornery

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Jan 13, 2021
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What tests have you performed while the vehicle is in the no-start condition?

Are you getting spark and fuel?
How old is the fuel?
What does the filter look like?

What do your plugs look like?
What's your coil input voltage while cranking?
Yes, I have spark at the coil and to the distributor, fuel is brand new spark plugs are all fine no fouling replaced the cables to each plug as well, the cap (yes I reset it to dead center) and the voltage is up to par on all points My next step is going to be testing the fuel pump to make sure it isn’t faulty and putting the appropriate psi and test all the vacuum. But I have this weird feeling the carb is just bad. Maybe I should completely replace the distributor as well rather than just innards... but at same time it’s got the right timing and spark lol. And the tests we tested all the points of spark both with ohms reader and the ol listen for the spark with a screwdriver lol
 

Noobz347

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Yes, I have spark at the coil and to the distributor, fuel is brand new spark plugs are all fine no fouling replaced the cables to each plug as well, the cap (yes I reset it to dead center) and the voltage is up to par on all points My next step is going to be testing the fuel pump to make sure it isn’t faulty and putting the appropriate psi and test all the vacuum. But I have this weird feeling the carb is just bad. Maybe I should completely replace the distributor as well rather than just innards... but at same time it’s got the right timing and spark lol. And the tests we tested all the points of spark both with ohms reader and the ol listen for the spark with a screwdriver lol
Hold off on that for just a tick.

There will be more dudes along with better questions, I'm sure. Carbed Mustangs are not a specialty of mine.

What strikes me odd though, is that your description indicates that it only happens when it's warm so...

Things that come to mind are your fuel pump. Yeah, getting a reading from the pump during cranking when it's cold/hot should help. Ignition components are always suspect when heat-soaked. Coil saturation and carb settings. :shrug:

You said you got backfiring. You also mentioned timing. It's also an old car. What kind of shape is the harmonic balancer in?

@General karthief Who's the resident carb expert?
 

Alextheornery

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Hold off on that for just a tick.

There will be more dudes along with better questions, I'm sure. Carbed Mustangs are not a specialty of mine.

What strikes me odd though, is that your description indicates that it only happens when it's warm so...

Things that come to mind are your fuel pump. Yeah, getting a reading from the pump during cranking when it's cold/hot should help. Ignition components are always suspect when heat-soaked. Coil saturation and carb settings. :shrug:

You said you got backfiring. You also mentioned timing. It's also an old car. What kind of shape is the harmonic balancer in?

@General karthief Who's the resident carb expert?
Haha yea.. it’s almost always been when it’s getting to a decent operating temp. That’s part of reason why I replaced those innards for the dist, as well as the ECU cuz I’ve read a lot that the durasparks are trash cuz they get faulty when it gets too hot. But even after those replacements, it’s still struggling. Plus the whole firing when stuff was sprayed into the carb but not staying running said fuel relations. Even tho you may not be an expert I thank you haha. I’ll double check the balance
 

Alextheornery

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Haha yea.. it’s almost always been when it’s getting to a decent operating temp. That’s part of reason why I replaced those innards for the dist, as well as the ECU cuz I’ve read a lot that the durasparks are trash cuz they get faulty when it gets too hot. But even after those replacements, it’s still struggling. Plus the whole firing when stuff was sprayed into the carb but not staying running said fuel relations. Even tho you may not be an expert I thank you haha. I’ll double check the
 

General karthief

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I don't know, I've forgotten what little I knew but I can fake it.
Make sure the choke flap is open all the way when warmed up.
Are the wires going to the control box, distributor in good shape?
Something changes when warm. So all diagnostic tests need to be done hot.
Fuel pressure and voltage
Float level (also check that the float in the carb actually floats)
Voltage at coil
 
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Alextheornery

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I don't know, I've forgotten what little I knew but I can fake it.
Make sure the choke flap is open all the way when warmed up.
Are the wires going to the control box, distributor in good shape?
Something changes when warm. So all diagnostic tests need to be done hot.
Fuel pressure and voltage
Float level (also check that the float in the carb actually floats)
Voltage at coil
Choke does fully open, wires going to should be good tho I will double check , both floats do float tho maybe I should check when hot and yea I got the right voltage at coil about 7700
 

Alextheornery

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Jan 13, 2021
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What's the voltage going into the coil during cranking and when hot?
I’ll have to check that actually, I think I only checked the old one and new one when it was cold. The old one was faulty giving wayyy too much voltage and compared it to the new one one when first installed
 

slow84lx

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You will be much better served with some troubleshooting and less part changing.

Have you replaced the coil recently? Is it a Motorcraft part? or a parts store part?

Is this car originally a carbed car or a conversion from CFI? Is the fuel pump electric or mechanical?

Did you verify that the harmonic balancer ring hasn't slipped when setting timing?

What carb is on the car? The original Holley 4180?

The wiring harnesses in these cars need to be checked very carefully when having these kind of issues.

Vacuum leaks will drive you crazy. Check all of the vacuum hoses / connections carefully.
 
Last edited:
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One of the biggest problems I have seen is to much fuel pressure at the carb. I run an Edelbrock carb and any pressure over 5 psi will push passed the seats and run rich. Factory manual pumps make around 9 psi and you must use a fuel pressure regulator to keep the pressure down. On a car that was fuel injected if your using the electric in tank pump the pressure can be 35-40 psi and after running for a bit the carb will look like a fountain spraying fuel out the top. Just something to check. Another question is what distributor are you running?
 

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Alextheornery

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Jan 13, 2021
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I just bought a regulator and a pressure gauge to test it. Butttttt I decided to look over specs of how basic tuning of the carb should be. My fathers been helping me out on this but he gets ahead of himself and starts tuning things with out adjusting the rest and says go for it. Sooo I got everything set to a normal tuning and I earlyr today I had noticed the primary jets were not spraying at all. So I adjust the floats cuz the primary was basically empty. Along with adjusting air flow ratio a bit I got the primary’s to spray again but that primary bowl keeps emptying out and not filling back up.
 

slow84lx

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What carb are you running?

Was this car originally a carbed, 5-speed car or a converted CFI & AOD car?

If you are still running the original Holley 4180 here is some good reading for you. Pay close attention to any posts from Walking Tall.


 
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Bree

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Jan 12, 2021
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I get the impression you're running an electronic fuel pump??? The "springs" in the fuel pump relay are probobly weak, When they get hot, they don't hold their position. It gradually worsens with time until it won't even start cold. I'd also check to see if a mousse or something chewed a wire under the relay panel.
 
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7991LXnSHO

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This really does not sound like a carb unless the choke is shutting when hot. That’s easy to check when you are waiting for it to cool off. A carb problem will more likely be there at start up or all the time. The filter fits the issues better, as does ignition components.
My hesitation in diagnosing it more specifically comes down to not knowing if your car is a stock, factory carb engine, a modified carb car, or a former EFI car.
When I put info from the two sites together, it looks like the carved, 210 Hp engines came with manual transmissions, and the EFI came with automatic. So what transmission do you have? That will help with the electrical side of the diagnosis.


Then, what numbers are on the carb choke horn? A picture of the carb would help in ID. It is very easy for someone to haves replaced the stock one with a 1850-x model or a newer street heat to get rid of the backwards idle screws or simplify tuning. Does it still have the EGR spacer plate under the carb and valve pointing to the back of the engine?
 
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